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VOLUME II 

Findings 



RINCON BAYOU DEMONSTRATION PROJECT 

Concluding Report 




UNITED STATES DEPARTMENT OF THE INTERIOR 
Bureau of Reclamation 

In Cooperation >^ith 

University of Texas Marine Science Institute 



September 2000 



Cover: Rincon Overflow Channel, September 1997 



RINCON BAYOU DEMONSTRATION PROJECT 



Concluding Report 



VOLUME II 



Findings 



September 2000 



Citation 

Bureau of Reclamation. 2000. Concluding Report: Rincon Bayou 
Demonstration Project. Volume II: Findings. United States 
Department of the Interior, Bureau of Reclamation, Oklahoma-Texas 
Area Office, Austin, Texas. 



Abstract 



The Rincon Bayou Demonstration Project significantly lowered the minimum flooding threshold of 
the upper Nueces Delta, thereby increasing the opportvinity for larger, more frequent diversions of 
fresh water from the Nueces River. During the 50-month demonstration period, the amount of 
fresh water diverted into the upper Nueces Delta was increased by about 732%. Five fireshwater 
inflow events were sufficient to activate the project's Rincon Overflow Channel and inundate, to 
varying degrees, the tidal flats of the upper delta. These tidal flows would not have otherwise been 
direcdy fireshened. As a result, in a relatively short period of time (only 4.2 years after the opening of 
the project's Nueces Overflow Channel), the average salinity gradient in the upper delta reverted to a 
more natural form, with average salinity concentrations in upper Rincon Bayou becoming the lowest 
in Nueces Delta. 

The effects of the demonstration project on the ecology of Rincon Bayou and the upper Nueces 
Delta were positive to the environment. Single-celled plant communities in the water column 
(phytoplankton) and on the surface of the sediments (microphytobenthos) evidenced increases in 
primary productivity with the reduction of salinity concentrations. Benthic communities (composed 
of bottom-dwelling organisms) evidenced increases in abundance, biomass and diversity. And, 
vegetation communities evidenced increases in plant cover and decreases in bare area. In summary, 
it was observed that freshwater inflow controlled, to a great extent, the ecological fiinction of the 
upper delta ecosystem by regulating critical biological mechanisms. 

A significant degree of ecological function was returned to the Nueces Delta and Nueces Estuary 
ecosystems by the demonstration project. Prior to the project, persistendy high salinity 
concentrations severely inhibited the function of the Nueces Delta, and the delta's natural 
contribution to the greater estuary ecosystem was limited to infrequent periods when natural flow 
events occurred. With the restored regular interaction between the Nueces River and Rincon Bayou, 
fresh water and nutrients were more consistendy introduced into the upper delta. As a resvilt, 
estuarine habitat in the delta component of the Nueces Esmary improved in both quality and 
quantity, and foraging opportunities for many estuarine species were increased. 



Contributors 



PRINCIPAL INVESTIGATORS 



Kenneth H. Dunton, Ph.D. 

University of Texas 

Marine Sdence Institute 

750 ChanneHview Drive 

Port Aransas, Texas 78373 



George H. Ward, Ph.D. 

Uruversity of Texas 

Center for Research in Water Resources 
University of Texas, PRC-119 
Austin, Texas 78712 



Paul A. Montagna, Ph.D. 

University of Texas 

Marine Science Institute 

750 Channelview Drive 

Port Aransas, Texas 78373 



Terty E. Whitledge, Ph.D. 

University of Alaska Fairbanks 
School of Fisheries and Ocean Sciences 
P.O. Box 757220 
Fairbanks, Alaska 99775-7220 



CO-AUTHORS 



Headier D. Alexander-Mahala 

University of Texas 

Marine Science Institute 

750 Channelview Drive 

Port Aransas, Texas 78373 



Richard D. Nelson, Ph.D. 

U.S. Bureau of Redamation 
Dakotas Area Office 
304 East Broadway Avenue 
Bismarck, North Dakota 58502 



Michael J. Irlbeck 

U.S. Bureau of Redaniation 

Oklahoma-Texas Area Office 

300 East 8th Street, Room 801 

Austin, Texas 78701 



Christine Ritter, Ph.D. 

Texas Water Development Board 
Environmental Section 
P.O. Box 13231 
Austin, TX 78711-3231 



Richard D. Kalke 

University of Texas 

Marine Science Institute 

750 Channelview Drive 

Port Aransas, Texas 78373 



Richard A- Roline 

U.S. Bureau of Reclamation 
Denver Center, Code D-8820 
P.O. Box 25007 
Denver, Colorado 80225 



Jonnie G. Medina 

U.S. Bureau of Redamation 

Denver Center, Code D-8510 

P.O. Box 25007 

Denver, Colorado 80225 



Dean A. StockweU, Ph.D. 

University of Alaska Fairbanks 
School of Fisheries and Ocean Sdences 
P.O. Box 757220 
Fairbanks, Alaska 99775-7220 



EDITOR 



Robert G. Harris 

U.S. Bureau of Reclamation 

Great Plains Regional Office 

316 North 26th Street 

Billings, Montana 59101 



PROJECT DIRECTOR 
Michael J. Irlbeck 



Contents 



Page 

1-1 Chapter One - Introduction 

1-2 Backgrovind 

1-4 The Rincon Bayou Demonstration Project 

1-4 Project Features 

1-9 Participants 

1-9 Authority and Funding 

2-1 Chapter Two - Study Area 

2-1 Background 

2-1 The Nueces Estuary 

2-2 The Nueces Delta 

2-3 Hydrography of the Nueces Estuary and Delta 

2-4 Water Level 

2-6 Salinity and Freshwater Inflow 

2-8 Ecology of the Nueces Estuary and Delta 

2-8 Estuarine Habitats in the Nueces Delta 

2-11 History of the Nueces Estuary and Delta 

2-14 Changes in Hydrography 

2-18 Changes in Ecology 

3-1 Chapter Three - Hydrography 

3-1 Introduction 

3-1 Objectives 

3-2 Methods and Approach 

3-2 Data Sources 

3-3 Quantification of Hydrographic Interactions in the 

Study Area 

3-4 Identification of Hydrographic Events During the 

Demonstration Period 

3-7 Results 

3-7 Overview of Hydrographic Events that Occurred 

During the Demonstration Period 

3-9 Summary of Selected Individual Events 



Contents V i 



3-20 


Discussion 


3-20 


Flooding Thresholds for the Upper Nueces Delta 


3-20 


Activation and Behavior of Demonstration Project 




Features 


3-22 


Summary 


3-23 


Exchange Events 


3-23 


Positive-Flow Events 


3-24 


Tidal Flat Inundation Events 


4-1 


Chapter Four - Water Column Productivity 


4-1 


Introduction 


4-1 


Objectives 


4-2 


Methods and Approach 


4-2 


Study Design 


4-3 


Measurements 


4-4 


Results 


4-4 


Hydrography 


4-5 


Nutrients 


4-10 


Phytoplankton Pigments 


4-12 


Water Column Production 


4-14 


Nutrient Amendment Bioassays 


4-18 


Microphytobenthic Sediment Biomass and Production 


4-22 


Discussion 


4-24 


Summary 


5-1 


Chapter Five - Benthic Communities 


5-1 


Introduction 


5-2 


Objectives 


5-2 


Methods and Approach 


5-2 


Study Design 


5-4 


Measurements 


5-6 


Results 


5-6 


Salinity 


5-7 


Temperature 


5-8 


Dissolved Oxygen 


5-8 


Benthos 


5-21 


Discussion 


5-21 


Macrofauna and Meiofauna 


5-22 


Trophic Links 


5-24 


Effects of Diversions as Disturbances 


5-24 


Summary 



ii ^ Contents 



Page 

6-1 Chapter Sex - Vegetation Communities 

6-1 Introduction 

6-2 Objectives 

6-2 Materials and Methods 

6-2 Monitoring Stations 

6-4 Open Water and Pore Water Chemistry 

6-4 Transect Sampling 

6-5 Percent Cover and Leaf Area Index 

6-5 Analyses 

6-6 Biomass 

6-6 Results 

6-7 Salinity 

6-12 Ammonium 

6-13 Nitrite+Nitrate 

6- 1 5 Large-Scale Whole Transect vVnalyses: Individual Species 

Responses to Events 

6-21 Large-Scale Whole Transect Analyses: Leaf Area Index 

6-21 Small-Scale Analyses 

6-37 Biomass 

6-43 Root: Shoot Ratios 

6-45 Discussion 

6-45 Salinity 

6-45 Inorganic Nitrogen 

6-46 Vegetation 

6-48 Summary 

7-1 Chapter Seven - Synthesis and Conclusions 

7-2 Changes in Hydrography 

7-2 Effects on Salinity 

7-5 Biological Responses 

7-5 Water Column Productivity 

7-6 Benthic Communities 

7-6 Vegetation Commumties 

7-7 Integration of Project Effects 

7-8 A Conceptual Model 

7-10 Summary 

8-1 Chapter Eight - Future Opportunities 

8-2 Opportunities for a Permanent Diversion Project 

8-2 Opportunities for Further Ecological Study 

8-2 Selection and Monitoring of Indicator Species 

8-3 Modeling 

8-4 Opportunities for Integration with Bay and Esmary 

Release Schedules 

8-4 Opportunities for Adaptive Management 

Contents 



9-1 


Chapter Nine - Literature Cited 


9-1 


Chapter 1 


Introduction 


9-2 


Chapter 2 


Study Area 


9-4 


Chapter 3 


Hydrography 


9-4 


Chapter 4 


Water Column Producti\'ity 


9-5 


Chapter 5 


Benthic Communities 


9-6 


Chapter 6 


Vegetation Communities 


9-8 


Chapter 7 


Synthesis and Conclusions 


9-8 


Chapter 8 


Future Opportunities 



Appendices 



A-l A Technical Notes ON THE RiNCON Gauge AND Data 

B-1 B Hydrography OF the Nueces Delta AND ESTUARY: 
1992-1999 

C-1 C Analysis OF THE Historic Flow Regime OF THE 

Nueces River into the Upper Nueces Delta and of 
THE Potential Restoration Value of the Rincon 
Bayou Demonstration Project 

D-l D Recent Trends IN Precipitation Occurring on THE 
Nueces River Watershed of South Texas 

E-l E Utilization OF EsTUARiNE Organic Matter During 
Growth and Migration byJuvenile Brown Shrimp 

PENAEUS AZTEOJS IN A SOUTH TEXAS ESTUARY 

F-1 F EFFECTS OF Temporality, Disturbance Frequency 
and Water Flow on an Upper Estaurine 
Macroinfauna Community 

G-1 G Field Notes AND Observations FROM Benthic 
Sampling Trips: October 1994 - December 1999 



iv ^ Contents 



Tables 

Tahk Page 

1 - 1 Summary of the annual monitoring program 

conducted as part of the demonstration project. 1-7 

2-1 Summary of mean annual flow of the Nueces River 
into the Nueces Estuary (1940 to 1996) and upper 
Nueces Delta (1940 to 1999). 2-18 

3-1 Summary of hydrographic data sources. 3-2 

3-2 Criteria used to define hydrographic events in the 

data record by response variables. 3-5 

3-3 Summary of hydrographic events which occurred 

during the demonstration period: October 1, 1994, 
through December 31, 1999. 3-8 

4-1 Hydrographic events (from Table 3-3) occurring 

prior to each water column sampling period. 4-6 

5-1 Hydrographic events (from Table 3-3) occurring prior 

to each benthic sampling period. 5-9 

5-2 Average benthos characteristics at all stations during 

the demonstration period. 5-9 

5-3 Parameters from nonlinear regressions to predict 

macrofauna characteristics from salinity (Figure 5-8). 5-13 
5-4 List of species average abundances over the course 

of the study at each station. 5-15 

5-5 Species overall dominance. 5-16 

5-6 Temporal species abundance (» individuals found 

in all 6 stations, 18 samples) per sampling period. 5-20 

5-7 Composition of the meiofauna community. 5-21 

6-1 Hydrographic events (from Table 3-3) occurring 

prior to each vegetation sampling period. 6-7 

6-2 Mean open water (OW) and pore water (PW) 

salinity values at each station. 6-8 

6-3 Mean open water ammonium (NH4*) and nitrite+ 

nitrate (NOj -I-NO3') concentrations at each 

station. 6-12 

6-4 Mean pore water ammonium (NH4*) and nitrite+ 

nitrate (NO2+NO3 ) concentrations at each 

station. 6-13 

6-5 Summary of species percent cover results from 

GIS analyses. 6-23 

6-6 Summary of LAI results from small scale 

GIS analyses. 6-38 

6-7 Average total biomass values for four halophjrte 

species at the three stations on each sampling date. 6-42 
6-8 Average (n=4) rootrshoot ratios for four halophyte 

species at the three stations on each sampling date. 6-44 

7-1 Summary of the effects of the demonstration project 

on the upper Nueces Delta. 7-8 

Contents 



Figures 

Figure Page 

1-1 Location of the Nueces Delta along the 

Coastal Bend of Texas. 1-2 

1-2 Historical population trend for the areas 

served by the Nueces River at Calallen. 1-2 

1-3 The Nueces River Basin, including major 

drainages and reservoirs. 1-3 

1-4 Study area for the Rincon Bayou Demonstration 

Project and location of project features. 1-5 

1-5 View of the Nueces Overflow Channel. 1-5 

1-6 View of the private road crossing separating the 

upper and central Rincon Bayou channels. 1-6 

1-7 View of the Rincon Overflow Channel. 1-6 

1-8 View of the low water crossing at the head of 

Rincon Bayou. 1-7 

1-9 Overview of the monitoring sites and stations 

established for the Rincon Bayou Demonstration 

Project. 1-8 

2-1 The Nueces Estuary and Delta. 2-2 

2-2 The Nueces Delta. 2-2 

2-3 Typical view of the upper Nueces Delta. 2-3 

2-4 Typical view of the lower Nueces Delta. 2-3 

2-5 Nueces River (looking downstream) just below the 

IH 37 bridge under flood conditions (April 1992). 2-7 

2-6 Natural flooding of the upper Nueces Delta, 

April 1992. 2-7 

2-7 View of salt marsh habitat in the upper Nueces Delta. 2-9 

2-8 View of water column habitat in central Rincon Bayou. 2-10 
2-9 View of muddy bottom habitat in upper Rincon 

Bayou during a period of low water. 2-10 

2-10 View of algal mat habitat near South Lake. 2-11 

2-11 Example of concrete rubble found at several points 
along the north bank of the Nueces River 
downstream from the IH 37 bridge. 2-12 

2-12 Example of Rangia middens (i.e., piles of bi-valve 

shells) found in Nueces Delta. 2-13 

2-13 Typical view of the Nueces River floodplain 

upstream of the Nueces Delta. 2-14 

2-14 The Nueces River Basin, including major drainages 

and reservoirs. 2-16 

2-15 Annual precipitation trends at four gauges about the 

greater Nueces River watershed since about 1900. 2-17 

2-16 Mean annual precipitation of available data at four 

gauges about the greater Nueces River watershed. 2-18 



Contents 



Piffi" Page 

3-1 Location of hydrographic gauging stations in the 

Nueces Delta and upper Nueces Bay. 3-3 

3-2 View of the Nueces River at Calallen Diversion Dam. 3-4 

3-3 View of the Rincon gauge (Station 0821 1 503). 3-4 

3-4 Hydrographic data for 1994. 3-10 

3-5 Hydrographic data for 1995. 3-11 

3-6 Hydrographic data for 1996. 3-12 

3-7 Hydrographic data for 1997. 3-13 

3-8 Hydrographic data for 1998. 3-14 

3-9 Hydrographic data for 1999. 3-15 

3-10 Selected hydrographic data for Events 12, 13 and 14 

(October 2 through November 3, 1996). 3-16 

3-11 Selected hydrographic data for Event 23 

(September 1 through 31, 1998). 3-18 

3-12 Discharge into the tidal flats area through the road 

crossing at the north end of the Rincon Overflow 
Channel during Event 16 (June 27, 1997). 13-19 

3-13 Visual satellite image showing the location of 
Hurricane Bret in relation to the Nueces 
Watershed just before landfall (August 22, 1999). 3-20 

3-14 The Nueces Overflow Channel looking southwest. 3-21 

3-15 The Rincon Overflow Channel. 3-22 

3-16 The low water crossing at the head of the upper 

Rincon Bayou channel. 3-22 

3-17 Typical view of the Nueces Overflow Channel 

during tidal exchange. 3-23 

3-18 View of the upper Rincon Bayou (background) 

during a typical positive-flow event (Event 16). 3-23 

3-19 View of diverted fresh water in the tidal flats area in 
the upper Nueces Delta during activation of the 
Rincon Overflow Channel (Event 16). 3-24 

4-1 Location of water column sampling stations in the 

upper Nueces Delta. 4-2 

4-2 Salinity at all water column stations (except 

Station 62) for each sampling date. 4-5 

4-3 Average salinity at each sampling site for each 

sampling date. 4-7 

4-4 Nitrate concentrations at all water column stations 

(except Station 62) for each sampling date. 4-8 

4-5 Ammonium concentrations at all water column 

stations (except Station 62) for each sampling date. 4-9 

4-6 Nitrate concentrations at all water column stations 

(except Station 62) for each sampling date. 4-9 



Contents V vii 



Figure Page 

4-7 Dissolved inorganic nitrogen concentrations at all 

water column stations (except Station 62) for 

each sampling date. 4-10 

4-8 Percent of dissolved inorganic nitrogen contributed 

by ammonium at all water column stations (except 

Station 62) for each sampling date. 4-11 

4-9 Phosphate concentrations at all water column stations 

(except Station 62) for each sampling date. 4-11 

4-10 Nitrogen to phosphorous (N:P) ratios at all water 

column stations (except Station 62) for each 

sampUng date. 4-12 

4-11 Chlorophyll concentrations at all water column 

stations (except Station 62) for each sampling 

date. 4-13 

4-12 Primary production at all water column stations 

(except Station 62) for each sampling date. 4-13 

4-13 Assimilation index at all water column stations 

(except Station 62) for each sampling date. 4-14 

4-14 Results from nutrient amendment bioassays for 

selected stations: March 7, 1997. 4-15 

4-15 Results firom nutrient amendment bioassays for 

selected stations: April 22, 1997. 4-16 

4-16 Results from amendment bioassays for 

selected stations: August 7, 1997. 4-17 

4-17 Sediment chlorophyll concentrations at all water 

column stations (except Stations 62 and 68) 

for each sampling date. 4-18 

4-18 Sediment chlorophyll to phaeopigments ratio 

at aU water column stations (except 

Stations 62 and 68) for each sampling date. 4-19 

4-19 Total sediment and water column chlorophyll 

and salinity. 4-19 

4-20 Sediment chlorophyll and water column 

chlorophyll. 4-19 

4-21 Sediment primary productivity' concentrations at 

all water column stations (except Stations 62 

and 68) for each sampling date. 4-20 

4-22 Sediment chlorophyU and sediment primary 

productivity. 4-21 

4-23 Sediment primary productivity and water 

column productivity. 4-21 

4-24 Total primary productivity (sediment and water 

column) and salinity. 4-21 

4-25 Assimilation index at all water column stations 

(except Stations 62 and 68) for each 

sampling date. 4-22 



Contents 



Pifft" Page 

5-1 Locations of benthic sampling stations. 5-3 

5-2 View of benthic Station C in upper Rincon Bayou 

under dry (above) and wet (below) conditions. 5-6 

5-3 Salinity at all benthic stations (A through F) for 

each sampling date. 5-7 

5-4 Marsh-wide average salinity at each sampling site 

for each sampling date. 5-8 

5-5 Dissolved oxygen at all benthic stations 

(A through F) for each sampling date. 5-10 

5-6 Average macrofauna biomass (a), abundance (b) 

and diversity at each station (A through F). 5-11 

5-7 Marsh-wide averages of macrofauna biomass, 

abundance and diversity for all stations. 5-12 

5-8 Relationship between average macrofauna 

characteristics and salinity. 5-14 

5-9 Stehlospio benedicti. 5-17 

5-10 Principal components analysis of 16 most common 

species, including species loadings (a) and station 

score plots (b). 5-18 

5-11 Average meiofauna abundance for each station (a) 

and marsh-wide (b). 5-19 

5-12 Marsh-wide average abundance of meiofauna 

and macrofauna. 5-21 

5-13 Comparison of marsh-wide average chlorophyll 

biomass with macrofauna biomass (a) and 

meiofauna abundance (b). 5-23 

6-1 Location of vegetation sampling stations in the 

upper Nueces Delta. 6-3 

6-2 The layout and dimensions of a typical vegetation 

transect. 6-5 

6-3 Total monthly precipitation during the demonstration 

period. 6-8 

6-4 Salinity for open water (a) and pore water (b) at each 

sampling site for each sampling date. 6-9 

6-5 Correlation between total flow through Rincon 

Bayou and open water salinity for each station. 6-10 

6-6 Mean pore water ammonium values for each station. 6-14 

6-7 Reference Station average total transect percent cover 
for the five dominant species and bare area on 
each sampling date. 6-16 

6-8 Station II average total transect percent cover for 
the five dominant species and bare area on each 
sampling date. 6-17 



Contents ^ ix 



Figure Page 

6-9 Station III average total transect percent cover for the 

five dominant species and bare area on each 

sampling date. 6-18 

6-10 Total transect leaf area index (LAI) for each sampling 

date at each transect. 6-22 

6-11 Reference Station percent cover maps for the five 

springtime sampling periods. 6-24 

6-12 Station II percent cover maps for the five springtime 

sampling periods. 6-25 

6-13 Station III percent cover maps for the five springtime 

samphng periods. 6-26 

6-14 Reference Station percent cover maps on the 

sampling date prior and three sampling dates 

following the July 1997 composite 

hydrographic event. 6-28 

6-15 Station II percent cover maps on the sampling date 

prior and three sampling dates following the 

July 1997 composite hydrographic event. 6-29 

6-16 Station III percent cover maps on the sampling 

date prior and three sampling dates foOowing 

the July 1997 composite hydrographic event. 6-30 

6-17 Reference Station percent cover maps on the 

sampling date prior and three sampling dates 

following the October 1998 composite 

hydrographic event. 6-31 

6-18 Station II percent cover maps on the sampling date 

prior and three sampling dates following the 

October 1998 composite hydrographic event. 6-32 

6-19 Station III percent cover maps on the sampling date 

prior and three sampling dates following the 

October 1998 composite hydrographic event. 6-33 

6-20 Reference Station percent cover maps on the 

sampling date prior and three sampling dates 

following the September 1 999 composite 

hydrographic event. 6-34 

6-21 Station II percent cover maps on the sampling 

date prior and three sampling dates following the 

September 1999 composite hydrographic event. 6-35 

6-22 Station III percent cover maps on the two sampling 

dates prior and two sampUng dates following the 

September 1999 composite hydrographic event. 6-36 

6-23 Reference Station leaf area index (LAI) firom 

GIS analyses. 6-39 

6-24 Station II leaf area index (LAI) from GIS analyses. 6-40 

6-25 Station III leaf area index (LAI) from GIS analyses. 6-41 



Contents 



7-1 Selected stations used for analysis of long-term 

salinity changes in Rincon Bayou. 7-3 

7-2 Long-term average salinity values for selected stations 
in the Nueces River and Rincon Bayou before (a) 
and after (b) implementation of the demonstration 
project. 7-4 

7-3 Total annual cumulations of selected freshwater 
sources affecting the Nueces Delta during the 
period 1992 through 1999. 7-5 

7-4 Conceptual model of the Nueces Delta ecosystem. 7-9 

8-1 View of the lower Nueces Delta with the City of 

Corpus Christi in the background. 8-5 




-S3U. 




^t^l^* .^^ 



Findings 



» -■ 



\\ 





CHAPTER ONE 

Introduction 



"These waters of the seaboard exert an 
influence upon the affairs of mankind far out 
of proportion to their size, for it is here that 
land and sea meet, and the fresh and salt 
waters of the earth intermingle." 

♦ E.J. Perkins 



The Rincon Bayou Demonstration Project was 
conducted from 1994 through 1999 within the upper 
Nueces Delta, located northwest of the city of Corpus 
Christi, Texas. The delta was formed and is supported 
by the Nueces River, which passes along its southern 
edge as it empties into Nueces and Corpus Christi bays 
(Figure 1-1). Because of the dynamic nature of this 
natural system, the delta consists of a variety of 
habitats, including open water, marshes and mud flats 
and possesses a unique array of hydrological and 
biological characteristics. 

Ecologically, the Nueces Delta is part of the greater 
Nueces Estuary, which is a brackish transitional zone 
situated between the freshwater riverine habitats of the 
Nueces River and the marine habitats of the Gulf of 
Mexico. Salt water from the bays regularly inundate the 
lower reaches of the delta during a variety of tidal and 
atmospheric events. The delta is also periodically 
inundated with fresh water when the Nueces River 
occasionally spills out of its banks. Such freshwater 
flooding events significandy contribute to the biological 
productivity of the delta by providing a medium for 
nutrient exchange and biochemical cycling, supplying 
fresh water to marsh plant communities, transporting 
detrital and other nutrient materials from the 
established marsh vegetation and sediments to the bay, 
and buffering bay salinity (Longley 1994). The Nueces 
Delta has therefore been considered one of the most 
important sources of nutrient material for the entire 
Nueces Estuary system (Texas Department of Water 
Resources 1981), supporting numerous plant and 
animal communities, including commercially important 
marine life. 



Chapter One ♦ 1-1 



BACKGROUND 



NUECES BAY 




DEMONSTRATION PROJECT 
STUDY AREA 



Aransas 
Bay 



Aransas 
Pass 



CORPUS CHRISTI BA Y 



RBDFISH 
BAY 



Approximate Scale 

Kilometers 

S 



OSOBAY 



Gult of Mexico 



lUarim ' %■■ 



Figure 1-1 : Location of the Nueces Delta along the Coastal 
Bend of Texas. 



Since the beginning of the 20th century, the human 
population along the Coastal Bend region of Texas has 
substantially increased (Figure 1-2). As a result, the 
demand for fresh water to meet the growing municipal 
and industrial needs of the region also increased 
significandy. In response, two large reservoirs were 
constructed in the Nueces Basin for the purpose of 
storing flood flows. The first was Wesley Seale Dam 
(Lake Corpus Christi), constructed on the Nueces River 
by the City of Corpus Christi in 1958, and the second 
was Choke Canyon Dam (Choke Canyon Reser\'oLr), 
constructed in 1982 on the Frio River by the 
U.S. Bureau of Reclamation (Reclamation) (Figure 1-3). 
Choke Canyon Dam was designed to be operated in 
conjunction with Lake Corpus Christi as part of one 
reservoir system. 

During the period when these two reservoirs were 
being planned, particularly in the case of Choke 
Canyon Dam, the potential adverse impacts of 
reservoir operations on the bay and estuary systems 
were a concern. It was generally suspected that these 
impoundments would reduce the amount of fresh 
water entering the Nueces Estuary and upper Nueces 
Delta, adversely affecting the natural productivity of 
these systems. However, there was very Utde specific 
information available regarding the needs of delta and 
estuary systems, or of their responses to changes in 



freshwater inflows. In recognition of the bay and 
estuary resources, the State of Texas required that, once 
Choke Canyon Dam was completed and filled, a total 



500 - 
















■D 

c 














1 


o 

S 300 












/ 


1 


Q. 

Q. 200 - 










/ 


/ 




o 

1- 

100 - 










/ 






- 








n 


f 







1860 1880 1900 1920 1940 1960 1980 2000 

Figure 1-2: Historical population trend for the areas 
served by the Nueces River at Calallen. 

Sources: City of Corpus Christi 1981 and Texas Water Board 
1992. 



1-2 V Introduction 




MEXICO 



Gulf 

of 

Mexico 



Figure 1-3: The Nueces River Basin, including major drainages and reservoirs. 

Source of topographic base map: U.S. Geological Survey 1997. 



of 186,274 10' m' (151,000 acre-ft) of water per year 
would be provided to the estuaries by a combination 
of releases and spills from the reservoir system at Lake 
Corpus Christi Dam and return flows to Nueces 
Estuary (Texas Water Rights Commission 1976). 
Although the dam was completed in 1982, flow into 
the reservoir was minimal for the first several years. 
However, in June 1987, record rainfall over the Frio 
River watershed filled the reservoir, and water was 
released as flood control for the first time. 

During the mid-1980's, several entities initiated regular 
monitoring programs in the region. Salinity in the 
Nueces Estuary began to be sampled continuously by 
several State agencies and universities. Fish, shrimp, 
crabs and oysters in Nueces Bay were sampled during 
routine. State-wide coastal inventory surveys. Modest 



research efforts were also undertaken by university 
researchers to monitor the effect of the water releases 
on hydrography and benthos in the estuary. 

By the early 1990's, it had become apparent that there 
had been a notable reduction in the freshwater inflow 
to the delta and esmary systems. For example, 
historically Nueces Bay had supported large popula- 
tions of shrimp and oysters, which generally require 
salinity concentrations in the range of 10 to 20 parts 
per thousand (ppt) salt. During the relatively dry 
period of the late 1980's and early 1990's, the salinity of 
the bay had increased to hypersaUne conditions 
(> 36 ppt), and consequendy the shrimp and oyster 
populations were reduced. For example, from 1984 
through 1989, shrimp har%'est declined in Nueces 



Chapter One ♦ 1-3 



Estuary, even though it was stable in the Aransas Bay 
ecosystem to the north and the upper Laguna Madre to 
the south (Montagna et al. 1998). 

Because of contention regarding the amount of fresh 
water dedicated to the bays and estuary from the 
reser\'oir system, releases had not been made for that 
purpose since Choke Canyon Reser\^oir filled. In 1992, 
the Texas Natural Resources Conservation 
Commission (TNRCC) implemented an operating plan 
requiring minimum mandator)' inflows on a monthly 
schedule totaUng 186,274 10^ m' (151,000 acre-ft). As 
part of the same order, TNRCC also created the 
Nueces Estuary Advisory Council to provide oversight 
of the releases and monitor the operating plan, and to 
make recommendations for improving the plan. In 
April 1995, the original TNRCC order was revised to 
adopt a "target" minimum monthly inflow plan instead 
of a mandatory inflow amount. This change, which 
resulted in an annual inflow target of 112,258 10^ m^ 
(91,000 acre-ft), was intended to mimic natural 
hydrographic conditions in the Nueces Basin while 
providing some relief to the water customers of the 
resen'oir system. 



THE RINCON BAYOU 
DEMONSTRATION PROJECT 

Beginning about 1993, a consortium of local, State and 
Federal entities began investigating alternatives to 
restore fresh water to the greater Nueces Estuary. In 
1993, as part of this initiative, the Reclamation 
undertook a temporary demonstration project to 
provide detailed scientific information regarding the 
freshwater needs of the delta and its response to 
changes in freshwater inflows. The Rincon Bayou 
Demonstration Project had two primary objectives: 

1) To increase the opportunity for natural 
freshwater flow events into the upper Nueces 
Delta; and 

2) To monitor any resulting changes in the hydro- 
graphy and biological productivity of the delta. 



Project Features 

The area selected for the demonstration study 
encompassed the northwestern portion of the upper 
Nueces Delta, or that area generally north of Rincon 
Bayou and west of the eastern-most railroad crossing 
(Figure 1-4). This area represents both the historic 
location of river inundation events and the western 
limit of the Nueces Estuary {i.e., the tidally influenced 
portions of Rincon Bayou and a large area of tidal 
flats). 

Water Diversion 

Reclamation decided on a final demonstration project 
design after reviewing several different alternatives. 
The selected alternative provided an uncomplicated 
means of increasing the opportunity for freshwater 
diversion and distribution in the upper delta, while 
preser%'ing the natural "event" mechamsm to which the 
system had adapted. The physical aspects of the 
project included two principal features: the Nueces 
Ch^erflow Channel and the Rincon Overflow Channel 
(Figure 1-4). 

The primary feature of the demonstration project was 
an overflow channel (Nueces Overflow Channel) 
excavated from the Nueces River to the headwaters of 
Rincon Bayou (Figures 1-4 and 1-5). The channel was 
located approximately 60 m downstream of the 
Interstate Highway 37 (IH 37) bridge along the north 
bank. The design dimensions of the Nueces Overflow 
Channel were approximately 274 m long and 12 m 
wide, with a bottom elevation of 0.6 m (2.0 ft) mean 
sea level (msl). This bottom elevation was selected so 
as to prevent regular tidal exchange between the 
Nueces River and the upper delta. However, early into 
the demonstration period, the channel's effective 
bottom elevation was lowered by flow events and tidal 
exchange to approximately mean sea level. Minor 
excavations were also made at two sites along the 
headwater channel of Rincon Bayou to remove 
channel-constricting sediment deposits, thereby 
allowing a higher diversion volume during flood events. 
Construction of the channel was completed 
on October 26, 1995. The purpose of the Nueces 
Overflow Channel and associated channel 



1-4 



Introduction 



Northern 
bluff line 




Calallen 
Diversion Dam 



Figure 1-4: Study area for the Rincon Bayou Demonstration Project and location of project features. The 

pre-existing private road crossing separating the upper and central Rincon Bayou channels was not part of the 
demonstration project. 




Figure 1-5: Viewof the Nueces Overflow Channel. The 

view is looking northeast, with the Nueces River in the 
foreground, and Rincon Bayou in the distant background. 
The photo was taken on June 26, 1997, during the first 
significant flow event. 

Photo courtesy of the Bureau of Reclamation. 



improvements was to lower the flooding tlireshold of 
the Nueces River, thereby increasing the opportunity 
for more frequent and higher magnitude flow events 
into the upper Nueces Delta. 

During the design phase of the project, Reclamation 
determined that an existing private road crossing over 
Rincon Bayou would act as a partial dam during larger 
flood events, limiting the volume of water diverted into 
die delta and substantially reducing the proposed 
project's effectiveness (Figures 1-4 and 1-6). There- 
fore, a second overflow channel (Rincon Overflow 
Channel) upstream of this road crossing was designed 
which would connect Rincon Bayou to the tidal 
mudflat areas in the northern part of the delta 
(Figure 1-7). 

The design dimensions of the Rincon Overflow 
Channel were approximately 610 m long and 30 m 



Chapter One 



1-5 




Figure 1-6: View of the private road crossing 
separating the upper and central Rincon Bayou 
channels. This pre-existing structure (constructed 
between September 1992 and March 1993) was not part of 
the demonstration project. The photo was tal<en on June 
26, 1997, during the first significant flow event. During 
larger events, the structure backed water up in the upper 
Rincon Bayou (left), as indicated by the difference in water 
levels on either side of the road. 

Photo courtesy of the Bureau of Reclamation. 




Figure 1-7: View of the Rincon Overflow Channel. The 

view is looking northeast from Rincon Bayou (foreground), 
showing the channel's outlet into the tidal flats 
(background). The photo was taken on June 26, 1997, 
during the first significant flow event. 

Photo courtesy of the Bureau of Reclamation. 



wide, with a bottom elevation of 1 .22 m (4.0 ft) msl on 
the upstream (south) end and 0.91 m (3.0 ft) msl on 
the downstream (north) end. In addition, the 
downstream end of the channel was crossed with an 
elevated road over eight 24'-diameter HDPE culverts. 
The acmal bottom elevations of the channel at the end 
of the demonstration period were about 1.14 m 
(3.75 ft) and 0.76 m (2.50 ft) msl, respectively. The 
primary purpose of the Rincon Overflow Channel was 
to provide a "spillway" during larger flow events that 
would divert floodwater around the private road 
crossing, thereby improving diversion and distribution 
of fresh water within the delta during larger events. 

In addition to these two principal features, numerous 
access road improvements were made as part of 
Reclamation's agreement with the pri%'ate landowners 
in the delta. These included the installation of cattle 
guards, placement of culverts in low areas, and 
rehabilitation of a low water crossing over the upper 
end of Rincon Bayou (Figures 1-4 and 1-8). This last 
feature was improved by raising the crest elevation 
from about 1.5 m (5.0 ft) msl to about 2.1 m (7.0 ft) 
msl, and adding thirteen 36"-diameter HDPE culverts 
in two locations to allow passage of flow events. The 
purpose of these road improvements was to preserve 
landowner access to the upper delta during the term of 
die demonstration project and to minimize the 
resistance to water moving into the delta during 
discharge events. 

Several other diversion alternatives were considered at 
the beginning of the study. These included: 1) either a 
total or partial diversion of the Nueces River into the 
delta, 2) delivery of a continuous flow of fresh water 
from the existing municipal infrastructure of either the 
San Patricio Municipal Water District or the 
O.N. Stevens Treatment Plant, 3) diversion of either 
river or groundwater through wind or solar pumping, 
or 4) some combination of the above. A detailed 
analysis of each of these alternatives was presented in 
the Plan of Study for the demonstration project 
(Bureau of Reclamation 1993). Each of these alterna- 
tives would have supplied some measure of fresh water 
into the upper delta and esmary. However, none of the 
alternatives would have adequately met the first 
objective of the demonstration project, which was to 



1-6 



Introduction 



restore the opportunity for natural freshwater flow 
events from the Nueces River into the delta. 

Monitoring Program 

The second objective of the demonstration project was 
to monitor any resulting changes in the hydrography 
and productivity of the delta from freshwater flow 
events. The data collection program was therefore 
designed to monitor hydraulic conditions in the study 
area, as well as those biological parameters that would 
be most responsive to project diversions: namely, 
water column productivity, benthic communities and 
vegetation communities. Each of these monitoring 
elements were regularly sampled at various stations in 
the upper delta (Table 1-1). The biological monitoring 
program was initiated in October of 1994, some 
12 months before the Nueces Overflow Channel was 
opened. This initial 12-month period served as a 
baseline period before the effects of the demonstration 
project began. The hydraulic monitoring began with 
the installation of gauging instrumentation in April of 
1996. In addition to the data direcdy collected as part 
of the demonstration project, the measurements of 
other monitoring and research efforts were also 




Figure 1-8: View of the low water crossing at the head 
of Rincon Bayou. This access road was one of several 
road improvements made as part of the demonstration 
project. The photo was taken on June 26, 1997, during the 
first significant flow event. 

Photo courtesy of the Bureau of Reclamation. 



Table 1-1: Summary of the annual monitoring program conducted as part of the demonstration project. 



Monitoring 
Element 



Response Variables 



Schedule 



Stations 



Hydrography 

Water Column 
Productivity 



Benthic 
Communities 

Vegetation 
Communities 



precipitation; and stage, velocity and calculated disctiarge through the 
Nueces Overflow Channel 

water quality (conductivity, temperature, depth, dissolved oxygen, 
calculated density, total suspended solids and water clarity) 

nutrients (orthophosphates, dissolved silicon, nitrate, nitrite, 
ammonium, particulate carbon and nitrogen (PC/PN), dissolved 
organic nitrogen (DON)) 

phytoplankton (species composition and size fractionation of major 
producing groups, biomass and growth rate of suspended species and 
of microphytobenthos (benthic phytoplankton)) 

macrofauna (species composition, density and biomass) 

meiofauna (species composition, density and biomass) 

pore water and open water (salinity, nitrate, nitrite, ammonium and 
temperature) 

macrophyte communities (species composition, percent cover and 
leaf-area) 

macrophyte communities (above-ground biomass, below-ground 
biomass and calculated root/shoot ratios) 



Continuous 


1 


Monthly 


8 


Monthly 


8 



Monthly 



Quarterly 


6 


Quarterly 


6 


Quarterly 


3 


Quarterly 


3 


Bi-annually 


3 



Chapter One ♦ 1-7 



utilized when available. These additional data sources 
included the Texas Coastal Ocean Obsen'^ing Network 
marine monitoring system of Texas A&M University- 
Corpus Christi Conrad Blucher Institute, the weather 
station network administered by the National Weather 
Service, the national stream flow gauging program 
conducted by the United States Geological Survey and 
unpublished data from faculty at the University of 
Texas Marine Science Institute. 



en\'ironmental conditions {e.g., tide, evaporation, 
precipitation, runoff, etc) as the treatment sites but 
would not be affected by the project's diversions. This 
site, located in the upper delta, ser^'ed as an acceptable 
"reference" to which data from treatment sites could 
be compared. Therefore, the demonstration study 
included a total of four monitoring sites: one reference 
and three treatment (upper Rincon Bayou, central 
Rincon Bayou and tidal flats) (Figure 1-9). 



The study "treatments" were considered to be 
freshwater diversions into Rincon Bayou either 
through the Nueces Gh^erflow Channel or over the 
bank of the Nueces River. A comparison site was 
needed that would be subject to the same general 



This mofiitoring aspect of the demonstration project 
involved the time obsen'ation of responses in the 
biological resources of the delta to intended (though 
not fully controlled) applications of fresh water. As 
with many biological studies, assessment of treatment 




NUECES : 

OVERFLOW 4' 

CHANNEL i^^ 

68^1 Rincon: 
gaugeT 

Hondo IH37 
Creek 



Calallen 
Diversion Dam 



^'ffincon Bayou 

central Rincon Bayou Site 
(treatment) 



Figure 1-9: Overview of the monitoring sites and stations established for the Rincon Bayou Demonstration 
Project. In general, the study design included one reference site and three treatment sites (upper Rincon Bayou, 
central Rincon Bayou and tidal flats). The location and number of sampling stations at each site is indicated by 
numerals (60 through 68) for water column productivity, letters (A through F) for benthic communities, and Roman 
numerals (I through III) for vegetation communities. 



Introduction 



applications is often complicated by the difficulty in 
establishing a suitable experimental control. Here, the 
establishment of a random control area was not 
practical. Consequendy, the basis of comparison of 
responses in biological productivity and species 
diversity to freshwater applications was fashioned 
according several considerations. First, hydrographic 
and biological data collected from the delta and 
surrounding area in years prior to the start of the 
current study offered some credible information on the 
status of the delta. Analyses of such pre-study data 
have been or are in the process of being published 
elsewhere, thus providing some characterization of 
conditions in the delta prior to the start of this study. 
Second, studies by scientists on this project and others 
have previously established some biological response 
relationships to fresh water in Gulf Coast estuaries, and 
in particular, responses to changes in salinity. These 
known responses were an important tool in assessing 
the effects of the demonstration project. The 
experience of project scientists with conditions in the 
Nueces Delta and similar estuaries offered highly 
relevant expertise to project studies. Third, a site was 
selected as a reference {i.e., comparison) point based on 
location and lower likelihood of impacts from 
freshwater inundations {i.e., treatments). Obviously, 
the more affected by project diversions, the less useful 
the reference site would be in evaluating treatment 
effects. Finally, the duration of data collection (over 
five years) and the varying freshwater inputs to the 
upper delta allowed the study of relationships to the 
varying biological responses {e.g., consistency in 
response). Consequendy, the demonstration project 
monitoring program was quasi-experimental in design. 
Nevertheless, previously established biological and 
ecological relationships applied here enabled the 
opportunity for deductive conclusions central to the 
goals of this study. 

In designing the monitoring plan for the demonstra- 
tion project. Reclamation again considered several 
different alternatives. Most of these alternatives 
included using data on wildlife or fish use of the area 
{e.g., mammals, reptiles, waterfowl, shorebirds, fin-fish, 
shellfish, etc.) as a measure of project success. 
However, considering the extensive non-estuarine 
migratory range of many of the species which utilize 



the study area, it was determined that these organisms 
would be subject to a variety of other forces not related 
to the demonstration project. Using such data to 
establish relationships between the direct effects of the 
project and observed changes in wildlife or fish 
populations or habitat use would have been difficult. 
For this reason, vertebrate wildlife use was considered 
to be an ineffective tool in evaluating the direct effects 
of the project on delta productivity. 



Participants 

There were several key participants in the Rincon 
Bayou Demonstration Project. The primary 
participants of the study, without whom this project 
would not have been possible, were the private 
landowners in the delta who granted Reclamation 
temporary permission to make the modifications and 
conduct the monitoring program. The design, 
coordination and funding of the project was provided 
by Reclamation. All of the biological data collection 
and analysis activities were conducted by researchers 
from the University of Texas Marine Science Institute, 
with support from the Texas Water Development 
Board. The hydraulic monitoring was conducted by 
the U. S. Geological Survey, who installed and main- 
tained the data collection equipment, and hydrographic 
data analysis was provided by the University of Texas 
Center for Research in Water Resources. 



Authority and Funding 

The Rincon Bayou Demonstration Project was 
conducted under the authority of the Federal 
Reclamation Act of June 17, 1902, as amended. 
Funding for the smdy was appropriated on a yearly 
basis by the United States Congress under 
Reclamation's General Investigations and Wetland 
Development programs. 



Chapter One ♦ 1-9 



CHAPTER TWO 

Study Area 



BACKGROUND 



The Nueces Estuary 



"If there are seventy-five square miles on this 
earth that disgrace it, those seventy-five square 
miles may be found here, Nueces Bay being 
one big slimy slough, only fit for the habitation 
of alligators and mud-snakes" 

♦ Dr.A.C. Peirce(1894) 



There is no universally accepted definition of an 
estuary, though these systems are generally considered 
to share the following properties: a 1) coastal water 
body, that is 2) semi-enclosed, with 3) free connection 
to the open sea, with both 4) an influx of seawater, and 
5) an influx of freshwater, and which is 6) of small to 
intermediate scale {e.g., Pritchard 1967; Ward and 
Montague 1996). The property of scale differentiates 
an estuary from larger systems, such as the 
Mediterranean Sea, the Baltic Sea, and the Gulf of 
Mexico, which satisfy the other properties but clearly 
are not estuaries. These properties do not necessarily 
occur all of the time, as in many estuaries, the relative 
influence of freshwater and seawater influxes varies 
with season. In essence, an estuary is a transitional 
system between a purely freshwater and a purely 
marine system. It is, therefore, influenced by processes 
that are terrestrial and marine, but there are also 
hydrographic features unique to the estuarine 
environment and a consequence of its transitional 
character. 



The boundaries of the Nueces Estuary include four bay 
systems (Texas Department of Water Resources 1982): 
one primary bay (Corpus Christi), one secondary bay, 
(Nueces), and two tertiary bays (Oso and Redfish) 
(Figure 2-1). In terms of geomorphic classification, the 
estuary is considered a coastal plain estuary (Pritchard 
1967), being composed of a drowned river valley lying 
perpendicular to the coastline. However, the Nueces 
Estuary also shares characteristics of lagoons with 
large, bar-built bays parallel to the coastline, like 



Chapter Two ♦ 2-1 



N,^--..,-, Nueces Delta 








^y?^— ^C^ 


Nueces Bay 


Redfish 
Bay 


i 




Corpus Chnsli Bay 












Approximate Scale 

Kilometers 

■ ■ ■ 
5 




Oso Bay 


Gulf 

of 

Mexico 



Figure 2-1: The Nueces Estuary and Delta. 

Redfish Bay. The total estuary has an average depth of 
about 2 meters (m) and covers approximately 
500 square kilometers (km^ (Orlando etal 1993). The 
bays are protected from the Gulf of Mexico by a 
system of barrier islands, and the only sigmficant 
tributary of the estuary is the Nueces River. As a 
transition between continental and oceanic environ- 
ments, the Nueces Estuary is subject to the effects of 



both marine and fluvial (riverine) elements. Included 
in the boundary of the Nueces Esmary are the tidally 
influenced portions of the Nueces Delta, which lies 
immediately west of upper Nueces Bay. 



The Nueces Delta 

The Nueces Delta (or Nueces marsh) is a complex area 
of vegetated marshes, mudflats and open water that 
covers approximately 75 square kilometers (km"^ 
(Figure 2-2). Along its northern boundary, the delta is 
separated from a large expanse of agricultural land by a 
steep bluff that reaches heights of about 20 m. To the 
south, the delta is separated from the municipal and 
industrial areas of the City of Corpus Christi by a 
similar bluff. The eastern limit of the delta is 
delineated by the upper segment of Nueces Bay, and 
the western limit by Interstate Highway 37 (IH 37) 
where it crosses the Nueces River. Upstream of this 



DEMONSTRATION PROJECT 
STUDY AREA 



Approximate Scale 
Kilometers 




Figure 2-2: The Nueces Delta. Generally depicted are open water (shaded) and tidally influenced areas. 

Source of base map: Salas 1993. 
2-2 ♦ Study Area 



point, the broad Nueces floodplain extends for several 
miles in a northwest direction, being confined to a 
width of approximately 2 to 5 km. 

Along the southern edge of the delta, the Nueces River 
flows west to east for approximately 15 km and 
empties into Nueces Bay. The banks of the river along 
this reach, which are generally about 1.5 to 2.5 m high, 
are steep and wooded. At higher flows, the river spills 
into the delta through numerous depressions along its 
northern bank. Within the delta itself, the two 
dominant hydraulic features are Rincon Bayou and 
South Lake (Figure 2-2). Rincon Bayou, which was 
likely once a course of the Nueces River, stretches 
along the entire northern portion of the delta to the 
bay. The depth and width of this channel varies gready 
along this reach, at times being confined to just a few 
meters wide and at other times opening up into several 
large, shallow lakes or pools {e.g., upper Rincon Bayou, 
central Rincon Bayou, and North Lake). South Lake is 
a large pool located in the south-central portion of the 
delta and is connected to the upper Nueces Bay by 
numerous tidal channels. 




Figure 2-3: Typical view of the upper Nueces Delta. 

The view is looking northeast, with North Lake in the 
background. 

Photo courtesy of the Bureau of Reclamation. 



brackish and saltwater marshes (Figure 2-4). Along 
both the nordiem and southern bluffs, remnants of 
diverse coastal forests exist where they have not been 
displaced by agriculture or development (Salas 1993). 



The delta is crossed latitudinally by two Missouri- 
Pacific (MoPac) railroads. The western-most railroad 
is located just downstream of the IH 37 bridge, 
crossing only the extreme western portion of the delta. 
The eastern-most railroad divides the delta roughly in 
half, with a majority of the lower (eastern) half being 
regularly inundated by water from upper Nueces Bay. 
The upper (western) half of the delta is generally higher 
in elevation, and only the lower channels and pools are 
regularly inundated by the bay. Both railroad crossings 
are elevated 3 to 5 m above tlie ground by fill material 
for most of their span, with the exception of a few 
bridged crossings over the more sigmficant channels. 

Generally, habitat feamres are quite diverse within the 
Nueces Delta, ranging from wooded upland areas to 
open bay waters. The western (upper) half of the delta 
is primarily dominated by rangeland and improved 
pastures, with brush thickets dominating the higher 
elevations, and a mixture of tntertidal mud flats, 
marshes, shallow pools and channels dominating the 
lower elevations (Figure 2-3). The eastern (lower) half 
of the delta is almost exclusively dominated by 



HYDROGRAPHY OF THE NUECES 
ESTUARY AND DELTA 

The Nueces Estuary and associated river delta are 
gready influenced by water originating from riverine. 




Figure 2-4: Typical view of the lower Nueces Delta. 

Nueces Bay is a short distance to the right, and the bluff 
along the northern boundary of the delta is in the distant 
background. 

Photo courtesy of the Bureau of Reclamation. 



Chapter Tivo ♦ 2-3 



estuarine and marine aquatic environments. There- 
fore, key hydrographic indicators like water level 
and salinity represent the combined effect of 
hydro-meteorological forces, including tides, wind, 
precipitation and local runoff, and river inflow. 



Water Level 

Water level variations in the Nueces Estuary are 
generally the result of interactions between 
(astronomical) tides, long-term secular excursions of 
the Gulf of Mexico and meteorological forcing (winds) 
(Ward 1997). 

Tides 

Of the many Fourier tidal components, four in 
particular account very well for the obser\'ed variation 
in the Nueces Estuary (\)C^ard 1997). The three 
astronomical tides, which are generally short-term 
influences, include a semidiurnal (12.4-hour) and 
diurnal (24.8-hour) tide, each of which is then 
modulated by a 27.2-day lunar tide (i.e., the variance of 
water level resulting from variations in the declination 
of the moon). The physical causes of fourth tidal 
component, a long-term secular variation in the Gulf 
of Mexico, are not well understood. In a "normal" 
year, there are clear maxima in the spring and autumn, 
and clear minima in winter and summer. The 
semi-annual variation is generally considered to be 
dominated by the winter minimum and autumn 
maximum (Chew 1964). Ward (1997) has summarized 
the featvures of this secular variation to include the 
following characteristics: 1) there are considerable 
year-to-year differences in the seasonal water-level 
variation, 2) the autumn maximum is usually the 
highest mean water elevation of the year and the winter 
minimum is usually the lowest, 3) the summer 
minimum in July and the autumn maximum in October 
are the most consistent in terms of seasonal regularity, 
4) both the winter (December to March) and spring 
(April through June) extremes have a considerable 
seasonal range in which they occur and can exhibit 
multiple extremes during these periods, and 5) despite 
reference to a semiannual period, the signal is not 
harmonic, as both the autumn maximum and the 



summer minimum, especially the former, tend to be 
more sharply focused in time and extend over two-six 
weeks in duration. 

This semi-annual variation is a prominent component 
of water level variation in the Nueces Estuary, and 
becomes increasingly important, compared to the other 
tidal components, with distance into the upper reaches 
of the bays, especially in Nueces Bay and Delta. The 
filtering properties of the inlets, shallow bays and 
channels gready attenuate the shorter period 
frequencies {e.g., semidiurnal and diurnal signals), but 
pass the longer periods with virtually no attenuation. 
As a result, water level variations caused by the cyclical 
lunar tide and the semi-annual secular rise and fall of 
water level in the Gulf represent the predominant long- 
term mechanism of water exchange with the Nueces 
Estuary. These two factors also account for most of 
the (gready attenuated) tidal variation of water level 
within the upper Nueces Bay and Delta. 

Wind 

On a shorter time scale, meteorological forcing (wind) 
is a prominent mechanism for water-level variation. 
The principal process is the response of a free surface 
to an imposed stress, referred to more colloquially as 
"setdown" or "setup" of water levels. Where there are 
adjoining bay systems (such as Nueces and Corpus 
Christi bays, or Corpus Christi Bay and the Gulf of 
Mexico), winds cause a direct downwind set-up of 
water levels across component bays. This difference in 
water levels causes a direct and an indirect water 
exchange between the water bodies. The direct effect 
is that of wind stress on the bay itself The indirect 
response of the same bay is that resulting from wind 
stress on the adjacent water body, which forces water 
into or out of the former. The area over which the 
winds operate and their duration are both important in 
the magnitude of the estuary's response. 

There are three types of meteorological forcing 
elements common to the Nueces Estuar^': frontal 
passages, sea breezes and storm surges. Frontal 
passages produce water level variations and 
accompanying transports of water. The primary 
mechanism is the change in direct wind stress on the 



2-4 ♦ Study Area 



water surface. As the front approaches the coastline, 
onshore wind flow is increased, setting up water levels 
along the coastline. With the frontal passage, winds 
turn abrupdy to the northern quadrant, reversing the 
direction of stress. Ward (1997) differentiates two 
classes of fronts: the relatively short-lived low-energy 
"equinoctial" frontal passages that do not force a 
response in the large water body of the Gulf, and the 
large-scale, longer duration "outbreak" fronts that 
result in exchange between the Gulf and the interior 
bays. 

The response of the Gulf of Mexico during frontal 
passages is the single most important factor 
determining the total response of the interior bay 
systems. For Nueces Bay there is a two-step response, 
the response of the bay to setdown of Corpus Christi 
Bay, and the response of Corpus Christi Bay to 
setdown in the Gulf of Mexico. The cross-bay 
transports are about the same magnitude for both 
equinoctial and polar-outbreak fronts. However, the 
volume of water exchanged is much greater for the 
polar-outbreak fronts since a response to the Gulf is 
involved (Ward 1997). The cross-bay transports, on 
the other hand, occur much more quickly but entail 
smaller water level changes and smaller volumes 
(generally on the order of 1% of the volume of the 
bay). 

Another short-period force affecting water levels in the 
Nueces Estuary and Delta is the sea breeze cycle. A 
sea breeze is a solenoidal circulation produced by the 
diurnal variation in density of the lower atmosphere 
resulting from the surface temperature differential of 
the land and sea (Haltiner and Martin 1957). It is 
ultimately caused by the difference in thermodynamics 
of sea water and land surface and is most pronounced 
along their boundary. As the sea breeze circulation 
begins to develop, it imposes an organized circulation 
in the lower atmosphere that spreads inland and 
increases the wind speed. The reverse circulation 
develops in the evening as a land breeze, spreading out 
to sea from the coastline. In the coastal zone itself, the 
sea breeze is manifested as a diurnal variation in wind 
velocity superposed on the normal onshore flow from 
the Gulf of Mexico. The familiar freshening of winds 
in the afternoon and the increase of short-crested 



wind-waves (chop) are well-known features of summer 
hydrography in these bays attending the sea breeze. 

The sea breeze is a relatively weak circulation, and its 
importance depends on other factors affecting wind. 
The effect of the sea breeze on water levels in the 
estuary may be minimized by more dynamic 
atmospheric processes, such as airmass replacement or 
interception of radiation by clouds and can be masked 
even by the prevailing onshore flow. The sea breeze is 
therefore best developed during conditions when these 
other influences are uncommon, which is typically 
during summer. 

Finally, the most extreme water level responses of the 
Nueces Estuary to meteorological forcing are 
associated with the storm surges of tropical 
depressions. These intense organized tropical systems, 
the most extreme representative being the hurricane, 
are characterized by a cyclonic circulation with intense 
swirhng winds around a center of extremely low 
pressure. The low pressure center and the circulating 
winds combine to create an elevated mound of water 
(storm surge) that moves with the depression, but the 
wind stress on the water's surface is the more 
important determinant of the magnitude of the surge. 

As the cyclone and associated storm surge make 
landfall, the volume of water in the surge behaves as 
any long-period shallow-water wave, slowing due to 
shoaling water depths and steepening. As the surge 
propagates into bays and estuaries, it is subjected to 
various local physiographic modifications. In some 
regions, this can lead to further amplification of the 
surge height, and some of the highest recorded surges 
on the Texas coast have occurred on the inland side of 
the bays, the largest being Hurricane Carla surge 
measured at Port Lavaca in excess of 6.7 m. While a 
hurricane can inflict damage through high winds, 
tornadoes and intense rainfall, it is the surge, perhaps 
in concert with wave attack, that is responsible for 
most of hurricane-related impacts on the Texas coast. 

In extreme instances, tropical storms may also have a 
secondary effect upon water levels in the estuary due to 
heavy precipitation. Such was the case with Hurricane 
Beulah, which made landfall in the autumn of 1967. 



Chapter Tm ♦ 2-5 



Inflow resulting from the record-setting flood event in 
the Nueces River caused a substantial rise in water 
levels in Nueces and Corpus Christi bays of up to 
0.6 m above that of the Gulf of Mexico (Grozier et al. 
1968; Corps of Engineers 1968). Grozier et al. (1968) 
went on to report that water levels in the bay were slow 
to fall (it took several days) because of the enormous 
amount of runoff from the rains inland, even though 
Corpus Christi Pass and two other new channels were 
opened between the bays and the Gulf by the storm. 



Salinity and Freshwater Inflow 

In general, salinity concentrations in estuary systems 
are stratified horizontally and vertically, both of which 
are primarily determined by the size and location of the 
freshwater source(s). Over the area of the estuary 
(horizontal), the gradient of fresh to brackish to marine 
salinity concentrations (which is to 1 5 to 30 parts per 
thousand (ppt) salt, respectively) progresses from the 
areas closest to the freshwater source {i.e., river's delta 
and mouth), to the secondary bay(s), then to the 
primary' bay. The degree of salinity stratification in the 
water column (vertical) depends upon the intensity of 
mixing. 

In Nueces Estuary 

As in most estuaries, there are substantial spatial 
gradients in salinity across the Nueces Estuary, not 
only because of the great range in hydro-chmatology, 
but also because of the location of the river drainages 
and the variable influence of the sea (Ward and 
Armstrong 1997). Based on evaluation of long-term 
data, seasonal variations in the salimty of the Nueces 
Estuary, other than a proclivity for sUghdy higher 
salinity concentrations in the summer, were not readily 
evident (Vt'ard 1997). Salinity variations within the 
water column {i.e., stratification) in the Nueces Estuary 
was found to be minimal (less than 0.5 ppt), but the 
largest values of this (small) gradient typically occurred 
in Nueces Bay, nearer to the freshwater source (Ward 
1997). One exception to this generalization is 
evaporation stratification, which t)'pically occurs in 
stagnant portions of Corpus Christi Bay during the 
summer. In this case, the salinity gradient in the water 



column can be as much as 6 ppt and often associated 
with hypoxia (Ritter and Montagna 1999). 

Short-term vacillations of salimt)^ values within the 
greater Nueces Estuary are primarily in response to 
water-mass changes. One of the most common and 
important contributors to this vacillation is the 
response of salinit)' to an influx of freshwater 
(extrusion) and the subsequent recover)^ of salinit)- as 
the inflow event diminishes (intrusion). These two 
mechanisms involve different physical processes, and 
therefore are not symmetnc events. Extrusion is 
effected by a replacement of water volume due to the 
rapid influx of the inflow (river) hydrography, 
commonly called a "freshet" event. Intrusion is 
accomplished by the internal circulations gradually 
returning higher salimty water to the upper reaches of 
the bay and delta. Extrusion typically occurs quickly, 
within a few days to several weeks (depending upon 
the region and the size of the freshet event), while 
intrusion typically requires weeks to months. Because 
the synoptic events producing the runoff are also 
frequendy accompanied by a frontal passage and 
regional rainfall, displacement of salt water by fresh is 
assisted by both wind forcing (frontal cross-bay 
transports and efflux to the Gulf) and surface 
precipitation surfeit. Intrusion, occurring over a longer 
time frame, is assisted by the t}'pical evaporation deficit 
at the surface. 

One factor of the regional hydro-chmatolog}' that 
facilitates identification of these kinds of events is that 
the largest freshets (which therefore have the potential 
for the greatest salinity response) are very widely 
spaced in time. Occasionally several such freshets 
occur closely enough in time that intrusion from one is 
superposed on extrusion from the other, making 
interpretation of the salinity response quite 
complicated. From a physical %'iewpoint, extrusion is 
an advective process, invoh'ing the wholesale 
displacement of water from the upper bay to the lower, 
or (for extreme events) into Corpus Christi Bay or 
even the Gulf of Mexico. Intrusion, on the other 
hand, is the combined effect of smaller water mass 
transfers, such as tidal exchanges and internal 
transports, whose cumiolative behavior is more of a 
diffusive process, operating to mix out and reduce the 



2-6 ♦ Study Ana 



overall salinity gradient. At any point in time, the 
salinity is a result of the combined effects of the two 
types of processes, but their relative importance 
depends upon the characteristics of the freshet 
response. 

In Nueces Delta 

In the Nueces Delta, variations in salinity 
concentrations of the pools, channels and marshes are 
driven by the same mechanisms as in the bays (i.e., 
water-mass exchanges invohring both intrusion and 
extrusion processes), with two sigtiificant differences. 
First, because much of the water in the delta is 
segregated into shallow pools and channels by higher 
land formations, many of these become frequently 
isolated from the bay due to water level fluctuations. 
This seclusion magnifies the effect of evaporation, 
resulting in the concentration of salts from bay water 
into the soils and water of the delta. The opportunity 
for continuous dilution by very large volumes of bay 
water enjoyed elsewhere in the estuary is not readily 
available in the delta. 

Second, the magnitude, frequency, duration and timing 
of freshwater flow events (or freshets) into the Nueces 
Delta are burdened by one additional condition not 
applicable to Nueces Bay. This condition is that the 
stage (water level) attained by the flood event in the 
river must exceed the minimum flooding threshold of 
the river in the delta segment of the stream 
(determined by the elevation and dimensions of the 
lowest portions of the river bank) (Figure 2-5). 
Therefore, unlike Nueces Bay, the opportunity for 
freshets in the delta are limited to discrete periods of 
time when the river hydrograph sufficiendy meets this 
condition. If the flooding threshold is not met, the 
flow event in the river will bypass the delta, providing 
Nueces Bay with an intrusion event without the same 
courtesy for the delta. 

These periodic deltaic inundation events usually coin- 
cide with tropical storm activity in early autumn or 
witii the passage of frontal systems in late spring 
(Texas Department of Water Resources 1982). Such 
flooding events flush the numerous channels and 
ponds of the delta, and inundate large areas to an 




Figure 2-5: Nueces River (looking downstream) just 
below the IH 37 bridge under flood conditions (April 
1992). During such high-flow events, the river spills fresh 
water into the upper Nueces Delta from several low points 
along the north (left) bank. 

Photo courtesy of the Bureau of Reclamation. 



extent and depth governed by the volume and duration 
of die flood event (Ward 1985) (Figure 2-6). These 
discrete events also serve as a mechanism whereby 
salts, as well as organic materials and other nutrients 
produced in the delta marsh, are exported from the 
delta into the greater Nueces Estuar)'. Because of 




Figure 2-6: Natural flooding of the upper Nueces 
Delta, April 1992. Such freshwater inundation events 
significantly contribute to the biological productivity of the 
delta by providing a medium for nutrient exchange and 
biochemical cycling, supplying freshwater to marsh plant 
communities, transporting detrital and other nutrient 
materials from the established marsh vegetation and 
sediments to the bay, and buffering bay salinity. 

Photo courtesy of the Bureau of Reclamation. 



Chapter Tm ♦ 2-7 



these events, the Nueces Delta is one of the most 
important sources of nutrient material for the entire 
estuary system (Texas Department of Water Resources 
1981). 



ECOLOGY OF THE NUECES 
ESTUARY AND DELTA 

Montagna et al. (1996) has described eight different 
t)^es of habitat subsystems occurring in the Nueces 
Estuary: salt marshes, beaches, the water column, 
muddy bottoms, sandy bottoms, oyster reefs, seagrass 
beds and algal mats. Although each subsystem is 
described separately, it is noted that there are many 
interconnections among them, as water cxirrents, waves 
and tides transport organic matter, energy and even 
animals pass between habitats (Montagna et al. 1996). 

According to a conceptual ecosystem model developed 
for the Nueces Estuary (Montagna et al. 1996), these 
eight habitat subsystems may be organized by their 
relative relation to the tidal water level. Intertidal 
habitats (those within the range of high and low tides) 
include salt marshes and beaches. Salt marshes are 
important sources of organic matter for the estuary, 
serve to buffer shorelines and provide habitat for 
important fish and wildlife species. Beaches support a 
low diversity of species because they experience high 
energy from waves, wind and currents which mix and 
transport detrital matter (plant and animal tissue) from 
the estuarine and marine environments. 

Subtidal habitats (those below the average low tide 
level) include the water column, muddy bottoms, sandy 
bottoms, oyster reefs and seagrass beds. The water 
column {i.e., the vertical portion of open water areas) 
supports a complex and productive food-web 
comprised of small, often single-celled organisms 
(plankton) that move with currents, and their 
predators, which are larger organisms (nekton), like 
fish. Beneath the water column, the substrate of the 
estuarine floor may be muddy or sandy. Muddy 
bottoms are more common in the estuary and support 
shrimp and other commercially important species, 
while sandy bottoms, which occur near the shoreline, 
are less common and support a number of large 



animals. Oyster reefs are associated with high diversity 
because they provide substrate, shelter and foraging 
opportumries for many different species. Seagrass 
beds are very diverse and productive, and serve as an 
important nursery ground for larval crustaceans, fish 
and invertebrates. 

Supratidal habitats (those above the average high tide 
level) include algal mats. Algal mats occur in isolated 
pools only periodically inundated by tide or rainfall and 
are comprised of nitrogen-fixing mats of filamentous 
blue-green algae living in colonies on the sediment 
surface. These habitats provide nutrients to shoreline 
environments and foraging habitats for wading birds. 



Estuarine Habitats in the Nueces 
Delta 

The most predominant estuarine habitat subsystems in 
the Nueces Delta are salt marshes, the water column, 
muddy bottoms and algal mats. The following more 
detailed description of these habitat types were 
primarily derived from Montagna et al (1996), except 
where noted. 

Salt Marsh 

Salt marshes are located in the shallow or intertidal 
regions of the estuary and delta, often near a source of 
freshwater input (e.g., river mouths and secondary 
bays), and are dominated by marsh grasses and other 
plants (Figure 2-7). The salt marsh of the Nueces 
Delta is the most extensive within the Nueces Estuary. 
Salt marshes trap soft sediment and organic material 
from the water column between individual plants. 
Beneath the plants are strong reducing conditions, and 
often low oxygen levels due to decomposition of 
organic matter. Areas with a higher frequency of 
freshwater inflow have higher diversity, higher rates of 
primary production and higher net community 
production. The biomass of producer and consumer 
organisms can also be high, but species diversity can be 
low because of fluctuating salinity. 



2-8 ♦ Study Area 




Figure 2-7: View of salt marsh habitat in the upper 
Nueces Delta. The vegetation transect for Station I is in 
this vicinity. 

Photo courtesy of the Bureau of Reclamation. 



Like seagrass beds, salt marshes are also an important 
nursery and feeding grounds for a variety of 
invertebrates and fish. Because of the amount of dead 
and decaying plant matter in salt marshes, their 
contribution of detritus to the food-web of adjacent 
habitats is important. The plant litter is uti]i2ed by 
micro-organisms and other small estuarine animals 
(Marples 1966) and serves as a critical link between 
primary and secondary trophic levels (Burkholder and 
Burkholder 1956; Odum and Wilson 1962; Teal 1962). 
Marsh plants also provide shelter for a variety of small 
organisms, including crustaceans, avians and mammals, 
and serve to stabiLi2e marsh sediments. Because of 
these functions, the marsh subsystem in the Nueces 
Delta supports numerous estuarine organisms, 
including shrimp, crabs and fin-fish, by providing large 
amounts of food and structure. 

One of the primary variables affecting the distribution 
and abundance of vegetation species in the delta marsh 
is salt. Salt is generally imported into the delta from 
Nueces Bay by a variety of tidal and wind forces, and 
generally exported from the delta by infrequent 
freshwater inundation events resulting from over- 
banking floods in the Nueces River. Water and soil 
salinity can be highly variable, depending upon 
precipitation, tide level and temperature (Henley and 



Rauschbauer 1981), especially because evaporation 
often exceeds precipitation for several months a year 
(Longley 1994). Elevation and proximity to channels 
and creeks are also factors that affect salinity, which 
often results in distinct vegetation zones (Chapman 
1960; Chapman 1974; Nixon 1982). 

Although most marsh plant species in the delta are salt- 
tolerant (halophytic), excessive salt concentrations can 
cause hypersaline conditions which are adverse to plant 
survival. For example, although halophytic species can 
survive in intermittent hypersaline environments, 
prolonged periods of salinity stress can stunt active 
growth and reproduction, leading to decreases in 
abundance and productivity (Deegan et al. 1986; 
Bertness ^/ «/. 1992). Ultimately, decreased vegetative 
coverage can create bare areas, which are then direcdy 
exposed to evaporation, leading to further increases in 
salinity (Zedler 1983; Bertness 1991). Successful re- 
colonization of bare areas requires at least a short-term 
lowering of salt concentrations in the soils, which 
allows re-invasion by vegetative (rhizomes) or 
reproductive (seed germination) growth. 

Water Column 

The water column habitats of the Nueces Estuary and 
Delta are shallow and are often only as productive as 
their substrate (Figure 2-8). This is in contrast with 
marine environments, such as the Gulf of Mexico, 
where the water column habitat is very deep and more 
productive than the bottom. Because fresh water 
mixes with salt water in the bays, the resulting salinity 
concentrations are often brackish (10 to 25 ppt). 
However, when evaporation exceeds freshwater inflow 
and flushing by the ocean, salinity concentrations can 
become saltier than the ocean (> 35 ppt). During dry 
periods in the delta, where supratidal pools and 
channels may become isolated for extended periods of 
time, salinity concentrations may exceed that of the 
ocean by several times. The estuarine water column is 
usually well oxygenated but can become quite turbid as 
sediment is re-suspended by wind or human actixdties. 
Mixing, due to the consistent high winds and shallow 
depths, prevents significant stratification of the bay 
water column. 



Chapter Tm ♦ 2-9 




Figure 2-8: View of water column habitat in central 
Rincon Bayou. 

Photo courtesy of the Bureau of Reclamation. 



The water column food-web consists of phytoplankton 
(single-celled plants) being eaten by zooplankton 
(single-celled animals), which in turn are eaten by fish. 
Primary production (the conversion of solar energy to 
chemical energy by photosynthesis) by phytoplankton 
in estuanne water can be relatively high but is typically 
much less than in salt marshes. There are two cycles of 
energy in the water column food- web. The first may 
be referred to as the "microbial loop," which includes 
only small flagellated phytoplankton and zooplankton 
and bacteria. Flagellates are a very diverse group of 
plankton that can travel short distances by beating their 
whip-Hke flagella. The small phytoplankton are preyed 
upon by small zooplankton, and, when the small 
phytoplankton and small zooplankton die, they may be 
decomposed by bacteria in the water. This food-web 
c\'cle is small, transfers energy rapidly, and its 
components are tighdy coupled. However, energy 
cycled in this microbial food-web are not usually 
transferred to laigher trophic levels (e.^., fish). 

The second food-web cycle of the estuarine water 
column consists of the larger phytoplankton, such as 
diatoms, which are eaten by zooplankton and some 
fish. In addition, some zooplankton are eaten by larger 
zooplankton and the lar\'al and adult forms of some 
fish. Even larger predatory fish (e.g., red drum, 
Sdaenops ocellatus) then eat these plantkivourous fish. 



This food-web cycle is larger, and it transfers energy 
slower than the microbial loop cycle, but both cycles 
are important in nutrient processing within the estuary. 

Open Water, Muddy Bottom 

The most common benthic habitat in the Nueces 
Estuar)- is the unvegetated muddy bottom (Figure 2-9). 
Movement of water over the surface of the mud keeps 
the sediment oxygenated to about one centimeter in 
depth. Below this region is a strongly reduced 
en\aronment due to the presence of ox)'gen-consuming 
microbial animals. Mud is easily re-suspended, and 
muddy bottoms may therefore experience frequent 
erosion or deposition of sediment. Turbidit)' tends to 
be high at the surface, which restricts the presence of 
light-dependant primarj' producers and filter feeders. 
Deposit feeders, however, can be present in high 
abundance, diversity and biomass. 

Detritus is the most important source of carbon for 
muddy bottom habitats. This material may originate 
from terrestrial sources transported by freshwater 
inflow, marine sources derived from marshes, 
seagrasses or sedimented phytoplankton. In the 
benthic muds, there are three types of animals which 
utilize detritus, including non-selective deposit feeders, 




Figure 2-9: View of muddy bottom habitat in upper 
Rincon Bayou during a period of low water. 

Photo courtesy of the Bureau of Reclamation. 



2-10 ♦ Study Area 



selective deposit feeders and omnivores. Non-selective 
deposit feeders (e.^., polychaetes) process bulk 
sediment by extracting organic matter from the mud. 
Selective deposit feeders (e.^., moUusks) usually have 
tentacles to pick and choose specific particles of 
material for ingestion. Omnivores (e.^., edible shrimp, 
Petiaeus sp.) eat detritus, microphytes or any small 
animals that they can catch. These benthic animals, in 
turn, provide prey items for many other estuarine 
animals, particularly fish. 

Algal Mat 

vMgal mats are unusual features of the supratidal zone 
that occur in some locations within the Nueces Estuary 
and Delta (Figure 2-10). They occur when rain, wave 
surges or higher tides collects in low spots near the 
shore, often in areas with higher elevation than salt 
marshes. The trapped water is very shallow, and often 
becomes quite warm and saHne with solar radiance and 
evaporation. However, these conditions allow a bloom 
of photosynthetic bacteria (cyanobacteria, or blue- 
green algae) that live on the sediment surface. These 
producers are very important to die bay ecosystem 
because they have the abiUty to fix atmospheric 
nitrogen (Nt) into forms more usable by other 
producers and bacteria like ammonia (NHj), nitrate 




Figure 2-10: View of algal mat habitat near South 
Lake. This mat is comprised of colonies of filamentous 
blue-green, unicellular green, flagellated and 
diatomaceous algae, bacteria and a minor assemblage of 
worms, crustaceans and ciliates. The vegetation is 
approximately 6 to 10 inches tall. 

Photo courtesy of U.S. Fish and Wildlife Service. 



(NOj) or nitrite (NO2). When this material is 
transported back into the bays, it represents a nutrient 
spike that can enhance primary productivity in the 
estuary. However, aside from the cyanobacteria, there 
are not many species endemic to the relatively harsh 
conditions of the algal mats. 

HISTORY OF THE NUECES ESTUARY 
AND DELTA 

Just over a century ago. Dr. A.C. Peirce, a Bostonian 
naturalist intent on collecting bird species from the 
coastal regions of Texas, traveled to the Corpus Chrisd 
area. At that time (about 1890), Corpus Christi was 
still a primitive setdement on the western bayfcont, 
accessible to the mainland by ferry across the Nueces 
near San Patricio, by fording El Rincon at Indian Point 
or by rail (Ward 1998). Peirce's book (1894) describes 
the travels of he and his guide (a local Corpus Christi 
resident and hunter), and includes several accounts of 
excursions into parts of the Nueces Estuary, 
particularly die Nueces Delta and upper bay. Although 
it reflects the values and judgements of the late 19th- 
century, the work pro\'ides a picture of the study area 
prior to many of the human activities that have 
changed it since: 

"About a week after my arrival at Corpus Christi,... 
we started for the Nueces Flats. Our road was mostly 
through a country covered with a low growth ofmesquite 
andweesatche [sic] brush, where pasture fences were 
much more numerous than houses, of which we saw 
few. . .. The land all about this part of the country is 
divided up into pastures containing many square miles 
each, which are occupied by thousands of sheep, goats, 
horses and neat cattle. . .. Twelve or fifteen miles from 
our starting place, we left the beaten road, and traveling 
four or five miles over a rough and hilly stretch of land, 
crossed the Nueces River and camped a few miles 
beyond 

"Above the function of the river with the bay is a large 
area of low marshy surface; this is the Nueces Flats, 
which include several thousand acres of land and water 
In hundreds of places on the north side of the river, the 
earth is depressed below the level of the stream; and 
these depressions, filled with water, are, in places, only 



Chapter Two ♦ 2-11 



separated from each other and the large stream by slight 
elevations. . .. As a rule, the bottoms of these small 
bodies of water are firm, but a few of those nearest the 
river an decidedly bogg)i. On each side of the river, and 
between the water and the grass-covered land, is a space 
perhaps twenty yards in width, which is made up of 
bottomless mud To venture on to this mud is simply to 
venture into it, and as it is seemingly without limit in 
depth, one might better try to walk on the ocean, so far 
as danger is concerned 

"We found wild geese and ducks in abundance; nearly 
every one of these small ponds was well stocked with 
them.... Gulls and terns were also plentiful. . .. These 
birds frequent the place in search of food, which they 
find about the strip of mud next the river. " (Peirce 
1894). 

From Peirce's description, the north bank of the 
Nueces River within the delta was very low. At 
present, the river bank in the upper delta is about 
1.5 to 2.1 m (5 to 7 ft) above that of the elevation of 
the river under low flow conditions. Several factors 
could have contributed to this change, including river 
channelization and the intentional deposit of fill 
material. Evidence of the later, in the form of 
scattered concrete rubble and re-bar, was found by 
Reclamation during construction of the Nueces 
Overflow Channel (Figure 2-11). 

Given the lower flooding threshold for delta 
inundations and the expanse and condition of the 
described mudflats adjacent to the river, the occurrence 
of freshets into the delta from the Nueces River were 
likely much more common at the turn of the century 
than at present. However, as can be observed today, 
the Nueces River in the delta was not continually fresh, 
as Peirce testifies that, 'ihe water of the river was not strong 
of salt, but was just brackish enough to fail completely to quench 
thirst" (18,94). That the Nueces River frequendy 
experienced dry or low-flow periods prior to 1900 was 
also observed by others (Hollon 1956; Collins 1878). 

Other evidence that the delta was much fresher than 
at present is the presence of Rangia middens 
(Figure 2-12). These piles of bivalve shells are the 




Figure 2-1 1 : Example of concrete rubble found at 
several points along the north bank of the Nueces 
River downstream from the IH 37 bridge. It is 

speculated that this material was intentionally placed in 
low portions of the bank to reduce flooding of adjacent 
pastures and was probably acquired from the highway 
bridge renovation during the late 1950's. 

Photo courtesy of U.S. Bureau of Reclamation. 



remains of foraging activities by Native Americans and 
may be found in the Nueces Delta at several locations 
along Rincon Bayou. Rangia cuneata is the dominant 
animal in Gulf coast estuaries where salinity 
concentrations continuously range from to 1 5 ppt 
(Hopkins et al. 1973). Although adults can sur\ave 
higher salinity values, lan^ae require concentrations in 
the range of 2 to 10 ppt for survival. The presence of 
large adult Bjingia in the Nueces Delta indicates that the 
habitat there had been primarily oligohaline. At 
present, adult Rangia are only found only in the Nueces 
River just below die Calallen Dam (Kalke 2000). 

Several weeks later, Peirce and his guide made a second 
excursion, only this time they ventured into the delta 
by boat from Nueces Bay: 



2-12 ♦ Study Area 




Figure 2-12: Example of Rangia middens {i.e., piles of bi-valve shells) found in Nueces Delta. This location 
is along the southern edge of the central Rincon Bayou channel. 

Photo courtesy of University of Texas Marine Science Institute. 



'From a man who lived by the abaters of Nueces Bay 
we rented a sail boat, at four bits a day. . .. Into this 
hay empties the Nueces River, and at the junction of the 
two is a mud flat, miles in extent The river is deep 
and narrow, but at its mouth spreads out, as it were, to 
cover this great surface with an inch or two of water. 
The amount of water over the flat depends in a great 
measure upon the wind; a hree;^efrom the east sending 
the water from the bay over the shoals, while a strong 
current of air from the west will have an opposite effect 
and leave the crest of the flat above the water's edge. 

"From various sources I learned that years ago the bay 
had extended several miles further back than now, and 
that the boggji soil on the sides of the river was at that 
time just such aflat as the one I have described If this 
is true there is no reason why the whole bay may not in 
time be replaced by land. Such a radical change as this 
is to be hoped for, for if there are seventy five square 
miles on this earth that disgrace it, those seventy five 
square miles may be found here, Nueces Bay being one 
big slimy slough, only fit for the habitation of alligators 
and mud-snakes " (Peirce 1984). 

Peirce's description of the dynamic rate of siltation in 
the delta indicates that there was, during that period, a 
very large amount of sediment was moving down the 
Nueces watershed and into Nueces Bay. In fact, 
Morton and Paine (1984) have reported that the 
shoreline of Nueces Bay had been accreting 



(advancing) into the bay for much of the period 
between 1867 xmtil the time of their research in 1982. 
White and Calnan (1990), who compared historical 
photographs of the lower Nueces Delta (below the 
eastern-most MoPac railroad) with those taken in 1959 
and 1979, also reported an increase in total delta area. 
Between 1930 and 1959, the total area of the lower 
delta (which included both vegetated area and barren 
flats) increased by 164 hectares (ha) (405 acres), and 
between 1959 and 1979, increased by 52 ha (133 acres) 
(White and Calnan 1990). This process of down-basin 
transport of sediment was also subjectively verified in 
the 1930's and 1940's, when La Fruta Dam, 
constructed in 1935 on the lower Nueces River, began 
to lose a significant amount of its storage capacity due 
to siltation (Corpus Chris ti 1990). 

However, although in recent years the total area of 
the lower delta has increased, the total vegetated area 
has decreased and the total water and barren areas 
have increased. During the 49-year period between 
1930 and 1979, the total area increased by 216 ha 
(534 acres), but the water/barren areas increased by 
345 ha (852 acres), for a net loss of 129 ha (318 acres) 
of vegetation (White and Calnan 1990). Subsidence 
(Brown etal 1976) and the loss of fluvial sediments 
from the Nueces River were identified as possible 
mechanisms causing the increase in water and 
barren areas and the loss of marsh vegetation 
(White and Calnan 1990). 



Chapter Two ♦ 2-13 



In yet another venture, Peirce and his guide traveled 
west from Corpus Christi by land, crossed the Nueces 
River from the south and entered into the upland 
Nueces floodplain several miles upstream from the 
delta: 

"Above the junction of the Nueces River with the bay 
the river is bordered on each side by a strip of timber 
several miles in width; the Nueces Bottoms. The 
bottoms are dark and gloomy, every particle of ground 
not occupied by the large and magnificent trees being 
covered with shrubs and tall palmetto leaves; while the 
direct sunlight is almost completely shut out by the long 
and flowing Spanish moss which covers every tree, 
weaving their twigs and leaves together in a tangled and 
matted web. But for the many roots which form a 
network beneath the surface, all this land would be too 
bogQi to uphold any living creature, and at the channel's 
sloping sides, where but few roots are present, it is 
dangerous to venture away from snags and logs. 

"Our course through the timber was anything but 
straight, and it was full two hours before the second river 
was before us. As was expected, here we met with 
trouble; for although the stream was narrow, it was not 
supplied with snags upon which we could cross. My 
partner predicted, that further down its course we should 
find the water-way much wider and more snagQi. 
Beating our way through the deep-tangled wildwoodfor 
two or three miles toward the bay, we reached that part 
of the swamp where the two rivers united at an acute 
angle. Here there was an entire change of scene. 
Instead of a narrow channel bordered by steep banks, 
there was a spread of mire acres in extent, wherein 
thousands and thousands of snags and water soaked 
logs were piled in confusion. More than this, hundreds 
of snakes were to be seen about and upon the driftwood, 
where they had come to bask in the sunshine and to feed 
in the shallow pools. Nearly everything supported at 
least one reptile, and I thought that before attempting to 
cross we would have to clear our intended path with 
powder and shot These serpents were not all of one 
species, but moccasins were the predominating kind " 
(Peirce 1894). 

From Peirce's description, the Nueces Delta at the turn 
of the century began where the heavily forested 



bottomland of the Nueces River ended. The present 
floodplain, however, is significandy less wooded than 
what Peirce observed (Figure 2-13), likely as a result of 
decades of agriculture and ranching activities. The 
"second river" encountered tn this floodplain was Hkely 
Hondo Creek, which at present is only a remnant braid 
of the lower Nueces that joins the river almost 
perpendicularly just downstream of Calallen Diversion 
Dam. The description of "thousands and thousands of 
snags and water soaked logs" at the upper end of the 
delta gives testimony to the size and effect of the 
immense floods that must have undoubtedly ravaged 
the heavily-wooded lower floodplain of the (reservoir- 
free) Nueces River watershed. 



Changes in Hydrography 

Water Level 

There have been numerous physical changes in Nueces 
Estuary during the 20th-century that have altered, to 
some degree or another, the historical response of 
water level to tides and wind. These modifications 




Figure 2-13: Typical view of the Nueces River 
floodplain upstream of the Nueces Delta. A flooded 
Hondo Creel< is visible in the lower right. The photograph 
was taken on June 26, 1997, during which a flood in the 
lower watershed had activated secondary channels in the 
floodplain. The telephone pole in the center of the 
photograph provides an approximate scale. 

Photo courtesy of U.S. Bureau of Reclamation. 



2-14 ♦ Study Ana 



include such activities as the stabilization and jettying 
of inlets, dredging of deep shipping channels in the bay 
and through the barrier island system, and installation 
of additional barrier features. Other factors include 
commercial shell dredging activity, which has been 
attributed for removing about 50% of the volume of 
Nueces Bay during the period of 1950 to 1968 (Ward 
1997), and the conversion of tidal marshes to upland, 
particularly along the south shore of Nueces Bay, 
which has transformed approximately 15% of the area 
of Nueces Bay since 1925 (Ward 1997). 

Salinity and Freshwater Inflow 

After compiling and analyzing salinity data throughout 
the Nueces Estuary from the 1950's through the 
1990's, Ward and Armstrong (1997) found diat the 
much of the estuary exhibited a well-defined increasing 
trend in salinity, including Nueces Bay and most of the 
open areas of Corpus Christi Bay. The average rates of 
increase were not considered insigmficant. For 
example, over ten years, the average rates of salinity 
increases would result in an increase of average saHmty 
of Corpus Christi Bay by 0.5 ppt and Nueces Bay by 
2.5 ppt (Ward and Armstrong 1997). After inspection 
of the data, these authors determined that the greatest 
contributor to the declining trend was the reduced 
frequency of occurrence of high-flow events in the 
Nueces River. Several factors have been identified as 
having possibly contributed to reduced stream flow in 
the Nueces River, including an increase in water 
storage and evaporation due to the construction of 
reservoirs, an increase in the consumption of water 
withdrawn from streams or shallow aquifers and a 
possible decrease in rainfall within the greater 
watershed (Asquith et al. 1997). 

Since 1935, three main-stem reservoirs have been 
constructed on the Nueces River and its tributaries. 
These dams include La Fruta Dam, Wesley Seale Dam 
(Lake Corpus Christi), which replaced La Fruta Dam, 
and Choke Canyon Dam (Figure 2-14). A direct result 
of their construction was that the combined basin 
storage capacity dramatically increased over a relatively 
short period from 67,848 10' m' (55,000 acre- ft) in 
1935 to over 1,221,264 10' m' (990,000 acre-ft) in 1982 
(Figure 2-14). Wesley Seale Dam lies on the lower 



reach of the Nueces River, and Choke Canyon Dam 
lies on the lower Frio River, the largest tributary to the 
Nueces. 

Developed for the purposes of providing a reliable and 
municipal water supply and flood protection, these 
dams have contributed to reduced streamflow in the 
lower Nueces River by their diminutive influence on 
larger river hydrographs, and through direct water loss 
to consumptive uses and evaporation. The present 
permitted firm yield of the reservoir system is 
171,470 10' m' (139,000 acre-ft) for mumcipal and 
industrial use, and a portion of delivered water returns 
to the estuary through treated return flows. Because of 
the relative shallow depth of the two reservoirs and the 
relatively hot climate, evaporation from these two 
water bodies can remove a significant amount of water 
from the river system. For example, during 1999 
alone, over 217,730 10' m' (176,500 acre-ft) were lost 
to evaporation from the combined reser\^oir system 
(Hilzinger 2000). 

Another factor possibly contributing to decreased 
stream flow in the lower Nueces River include 
increased non-reservoir surface water withdrawals in 
the greater watershed. For example, long-term (1940 
to 1990) analysis of reported surface water withdrawals 
in the basin upstream of the reservoirs indicates an 
increase of about 60 % from 1965 to 1990 (Green and 
Slade 1995), which could have reduced the amount of 
inflow to the reser\^oLrs. 

Finally, a decreasing precipitation trend in the Nueces 
watershed would be expected to reduce streamflow. 
Howerver, after analyzing rainfall data from four south 
Texas gauges (Cotulla, Beeville, Sabinal and Corpus 
Christi) reflecting conditions for the Nueces watershed, 
Medina (2000) found that annual precipitation (using a 
base period that consisted of data since 1900) 
produced no particular trend (Figures 2- 1 5a through 
2-1 5c). Using a baseline that began during the late 
1940's {e.g.. Figure 2-15d), annual precipitation 
portrayed an increasing trend (Medina 2000). The 
most prominent and common feature of the 
precipitation data at all stations was the drought of the 
late 1940's and early 1950's. Similarly, Asquith etal. 



Chapter Two ♦ 2-15 




Figure 2-14: The Nueces River Basin, including major drainages and reservoirs. 

Source of topographic base map: U.S. Geological Survey 1997. 



(1997) found little evidence for statistical trends in 
precipitation along the Coastal Bend from 1968 
through 1993. 

Recendy, two independent investigations were 
conducted to quantify' changes in streamflow of the 
Nueces River since 1940, one regarding inflow into 
Nueces Estuary and the other regarding inflow into 
Nueces Delta. The first study was conducted by 
Asquith et al. (1997), who used streamflow in the 
Nueces River near Mathis (located immediately below 
Lake Corpus Chrisd) to generally represent estuary 
inflow. After performing a statistical trend test on 
daily flow values, it was concluded that the data 



showed strong exddence for a downward trend for the 
period of 1940 to 1996. When analyzed in the 
historical context of reservoir construction, it was 
reported that the change in mean annual streamflow 
during the period after the construction of Lake 
Corpus Christi (1958 to 1982) was negligible (a 
decrease of about 1%), but that the change during the 
period after construction of Choke Canyon Dam (1982 
to 1 996) was large (a decrease of about 55%) (Asquith 
et al. 1997). Although their analysis used the 
construction dates of large reser\'oirs to delineations of 
the record, it was explained that the effects of 
reservoirs were only partially responsible for the 
observed differences between the time periods. 



2-16 ♦ Study Ana 



(a) Cotulla 

50| 1 1 1 1 1 1 1 1 r 



40 



c 
o 



ra 30 



20 



10 




P^^<i^^^<i^^^<ii^^<i^^^<i^^^#VVVV^ 



(b) Beeville 



50ri — I — I — I — I — I — I — I — r 



20 




10 



J I I I I I L 



^<i^^^<i^\<i'V^^<iiV''^<i^^^<J^^^<i\^^^*VV'' 



(c) Sabinal 

50| — I — r- 



40 



10 



1 — I — I — I — r 




• • • 

J I I I 



,<iO°^<,^^^<,^\<J^,<,^V'',<i^^^<j^^^#^<i<^V^ 



(d) Corpus Christi 

50| — I— 



40 



ra 30 



Q. 

'o 



20 



10 



1 1 1 1 1 1 r 




I I L 



,<4^\<i^W\<^^\<i^\'*^\'i'^V\'*"^V 



i 



Approximate Scale 

Kilometers 

50 




• Beeville 



Figure 2-15: Annual precipitation trends at four gauges about the greater Nueces River watershed since 
about 1900 Source: Medina 2000. 

Note: 1 inch = 2.54 cm 



Chapter Tm ♦ 2-17 



The second study was conducted by Irlbeck and Ward 
(2000), who used river over-banking events into the 
upper Nueces Delta (i.e., Rincon Bayou) to generally 
represent delta inflow. After analyzing the flow regime 
characteristics of deltaic inundation events, it was 
concluded that the magnitude of such events has also 
decreased during the period of 1940 to 1999. Again, in 
the context of reservoir construction, it was reported 
that change in the annual mean flow into the delta 
from the river during the period after the construction 
of Lake Corpus Christi (1958 to 1982) was large (a 
decrease of about 39%), and that the change during the 
period after construction of Choke Canyon Dam (1982 
to 1999) was very dramatic (a decrease of over 99%) 
(Irlbeck and Ward 2000). 

The comparative results of these two investigations 
(Table 2-1) show a marked difference in the response 
of the Nueces Delta to reductiotis in stream flow over 
the past sixty years when compared to that of the other 
two periods (1958 through 1981, and 1982 through 
1999) (Figure 2-16). The distribution of large 
precipitation events were also found to be less frequent 
during the drought years of the late 1940's and early 
1950's than in the latter two periods, which were not 
significandy different from each other (Medina 2000). 

This information is relevant because this first period, 
which was used as the baseline for calculating percent 
changes in estuary and delta inflow by Asquith et al. 
(1997) and Irlbeck and Ward (2000), respectively, likely 
under-represents to some degree the actual baseline 
conditions for freshwater inflow to the Nueces Estuary 
and Delta. In addition, the second period, which 




1940-1957 1958-1981 1982-1999 

H Sablnal H Beeville 5 NE 

n Corpus Christi CD Cotulla 



Figure 2-16: Mean annual precipitation of available 
data at four gauges about the greater Nueces River 
watershed. Source: Medina 2000. 

Note: 1 inch = 2.54 cm 



showed the smallest percent change in both analyses, 
also experienced the largest mean watershed 
precipitation (Figure 2-16). 



Changes in Ecology 

The history of the Nueces Estuary is not dissimilar 
from that of many other estuaries, in that it includes 
man-made alterations such as diverting or removing 
freshwater sources, creating channels, extracting 
sediments, disposal site of waste materials and 
harvesting plants and animals. These changes have 



Table 2-1 : Summary of mean annual flow of the Nueces River into the Nueces Estuary (1 940 to 1 996)' and upper 
Nueces Delta (1940 to 1999)^ Time periods in both studies were based upon the construction dates of large reservoirs in 
the watershed. 



Time Period 


Mean annual river flow 

into Nueces Estuary 

(acre-ft) 


Percent change 
from Period 1 


Me< 
into 


m annual river 

upper Nueces 

(acre-ft) 


flow 
Delta 


Percent change 
from Period 1 


1940-1957 
1958-1982 
1983-1996(9) 




619,000 
614,000 
279,000 


-0.8% 
-54.9% 




127,997 

77,989 

537 




-39.1% 

-99.6% 


' Source: Asquith ef a/ 1997. 
^ Source: Irlbeck and Ward 2000. 








Note: 1 


acre-ft =1.2336 10' m' 



2-18 ♦ Study Area 



often greatly affected the general ability of the estuary 
to offer the diverse range of habitats that allow a wide 
range of organisms to thrive. Major alterations 
affecting water-mass exchanges between the estuary 
and the Gulf (and therefore the response of water 
levels in the Nueces Estuary and Delta) include 
alterations of the Nueces River channel, oyster shell 
mining and the excavation of shipping channels, barrier 
structures and artificial passes. The primary alteration 
affecting salinity concentrations in the estuary and delta 
is reduced freshwater inflows from to a combination of 
dam construction and water extraction for agriculture, 
industry and municipal uses. 

Data records do not exist to accurately document all of 
the resulting alterations to the ecology of the Nueces 
Estuary and Delta over the past century. However, it 
is generally agreed that the combined effect of these 
activities has resulted in an estuary and delta with 



higher salt concentrations in the soils and water, 
channelized flow that reduced flooding by freshwater 
overflow and tides, species shifts to more salt tolerant 
forms and probably reductions in the total number and 
biomass of plants and animals. The most notable and 
evident changes have been reductions in oyster and 
shrimp harvests (Montagna et al. 1998), each of which 
requires a low salinity regime at some point in their life 
cycle (Moffet 1970). With few exceptions (e.g., the 
monitoring work conducted by the Texas Parks and 
Wildlife Department in Nueces Bay), available data has 
been primarily derived from subjective comparisons of 
historical sources with present conditions, inferences 
drawn from similar ecosystems, which have endured 
comparable change processes, and direct observations 
from recent monitoring activities. 



Chapter Tmo ♦ 2-19 



GEORGE H. WARD 

Center foi Research in Water Resources, 
University of Texas, Austin 

MICHAEL J. IRLBECK 

U.S. Bureau of Reclamation, Austin 



CHAPTER THREE 

Hydrography 



INTRODUCTION 



"Water is the blood of the earth, 

and it flows through its muscles and veins. 

♦♦♦ Kuantzu (late 4th Century) 



The Nueces Estuary and Delta are subject to numerous 
hydro-meteorological forces, including a combination 
of river flow, precipitation, peripheral runoff and 
forcing from wind and tide. Each of these influences, 
in isolation or in combination, may gready affect the 
quality and quantity of estuarine habitat available. Such 
hydrographic events can alter water chemistry [e.^., 
salinity and nutrients); transport detrital material; cause 
large volume exchanges between the bay, delta and 
river; and make accessible or restrict habitats available 
for estuarine aquatic organisms. Therefore, 
interpretation and analysis of the effects of the 
demonstration project and resulting biological 
responses required a comprehensive understanding of 
the hydrography of the area during the study period. 



OBJECTIVES 

1) To quantify (by direct measurement and data 
analysis) the hydrographic interactions of the 
upper Nueces Delta with the Nueces River and 
Nueces Bay during the study period (October 1, 
1994, through December 31, 1999); 

2) To identify and describe significant hydrographic 
events during this period; and 

3) To describe the observed changes in the hydraulic 
characteristics of Rincon Bayou resulting from the 
demonstration project features. 



Chapter Three ♦ 3-1 



METHODS AND APPROACH 
Data Sources 

Hydrographic data for this analysis were obtained from 
a variety of sources, including the Texas Coastal Ocean 
Observing Network (TCOON) marine monitoring 
system of Texas A&M University-Corpus Christi 
Conrad Blucher Institute (CBI), the weather station 
network administered by the National Weather Service 
(NWS) and the national stream flow gauging program 
conducted by the U.S. Geological Survey (USGS) 
(Table 3-1). 

Table 3-1: SummatY of hydrographic data sources. 



surface in Nueces Bay is in response to meteorology, 
tides and river hydrographs, this is negligible in 
comparison to the temporal excursions in water level 
in the river and marsh. Therefore, stage data from the 
White Point gauge was regarded as an acceptable 
indication of the general coincident elevation of 
Nueces Bay. 

The USGS gauges from which data were obtained 
included the Nueces River at Calallen (Station 
0821 1500) (Calallen gauge) and Rincon Bayou near 
Calallen (Station 08211503) (Rincon gauge). The 
Calallen gauge was located on the Nueces River, about 
0.64 km upstream from Calallen Diversion Dam 



Data Source 



Parameter 



Measurement Location 



Data Type 



Nueces River at Calallen, 
USGS 

TCOON system, CBI 



Corpus Christi Bay NWS 

Rincon Bayou near Calallen, 
USGS 



Estimated daily flow 

Water level 

Wind direction and velocity 

Salinity 

Daily precipitation 

Water level 
Current velocity 
Calculated flow 

Daily precipitation 



Nueces River 

Nueces and Corpus Christi 
bays 

Corpus Christi International 
Airport 

upper Rincon Bayou 



Data recorded at 
15-minute intervals 

Data recorded as 
6-mlnute averaged 
values 

Data archived as daily 
values 

Data recorded at 
15-minute intervals 

Data archived as daily 
values 



Data obtained from the TCOON system included 
salimty and water level. For each of these parameters, 
hourly measurements were obtained from the CBI data 
archive, and, for the analysis reported here, subjected 
to 24-hour averaging to obtain daily mean values. The 
salinity data used were from the CBI SALT03 gauge, 
which is situated due south of VCTiite Point in the 
center of the bay about equidistant from the mouth of 
Rincon Bayou and the mouth of the Nueces River 
(Figure 3-1). This gauge responds to flow from both 
conveyances. Salinity concentrations are measured by 
robot conductivity sensor and converted to salinity, 
reported in parts per thousand (ppt). 

Water level in Nueces Bay was that measured at CBI's 
White Point gauge, nearer the mouth of Rincon Bayou. 
While there is no doubt some slope to the water 



(Figure 3-2) and about 3.05 km upstream from the 
entrance of the Nueces Overflow Channel. Flow data 
were obtained from both gauges, and stage and 
precipitation data were also obtained from the Rincon 
gauge. There are significant limitations to the accurac)' 
of the flow data from the Calallen gauge at higher 
values for two primary reasons. First of all, the gauge 
ceases to represent the total flow of the Nueces River 
above about 56.63 cubic meters per second (m'/s) 
(2,000 cubic feet per second (cfs)) due to activation of 
additional flow channels in the floodplain. Second, no 
reliable field observations of discharge values are 
available above 77.87 m'/s (2,750 cfs), so all daily flow 
values in excess of 77.87 m'/s (of which there were 
3 in the record under review) were estimated by 
extrapolation. 



3-2 



«* 



Hydrography 



DEMONSTRATION PROJECT 
STUDY AREA 



Approximate Scale 
Kllomelers 




Figure 3-1: Location of hydrographic gauging stations in the Nueces Delta and upper Nueces Bay. The location of 
the Corpus Christ! Airport station is not shown, but is located approximately 14 l<m to the southeast of the study area. 



The Rincon gauge was located in the head-water 
channel of Rincon Bayou, approximately 274 m 
downstream (Figure 3-3). There was a gap in the data 
record from the Rincon gauge from October 26, 1995, 
when the Nueces Overflow Channel was opened, 
through May 15, 1996, when the Rincon gauge was 
activated. The absence of data limited the analysis of 
the demonstration project during this seven-month 
period when no measurements of flow, water level or 
precipitation at the station were available. However, 
because this period proved to be relatively dry with few 
hydrographic events, the missing data were not deemed 
critical. Additional technical information on the 
Rincon gauge instrumentation, data and observed rela- 
tions with the Calallen gauge was developed separately 
by Irlbeck and Ockerman (2000) and has been included 
as Appendix A of this Concluding Report. 
Precipitation data were also obtained from the 
National Climatic Data Center records for Corpus 
Christi Airport, which is located approximately 



14 kilometers southeast of the study area. This gauge 
supplied precipitation data for the region before the 
Rincon gauge was installed. 



Quantification of Hydrographic 
Interactions in the Study Area 

Once the hydrographic parameters of interest were 
thus identified, raw data for the period of 1992 through 
1999 were obtained from these sources and compiled 
by Ward (2000). Prior to late- 1993, virtually the only 
extant data for the study area was salinity in Nueces 
Bay and inflow measured at Calallen. In 1993, reliable 
records from CBI tide gauges and anemometers 
became available. By the date of the breaching of the 
Nueces Overflow Channel on October 26, 1995, fairly 
continuous data from the region was available, with 
only the USGS gauge in Rincon Bayou itself lacking. It 
did not become operational until May 16, 1996. 



ChapterThm ♦ 3-3 




Figure 3-2: View of the Nueces River at Calallen 
Diversion Dam. The photo was taken on June 26, 1997, 
for which the mean daily flow rate in the river was 
63.7 mVs (2,250 cfs). 

Photo courtesy of the Bureau of Reclamation. 



Identification of Hydrographic 
Events during the Demonstration 
Period 

In an effort to develop a common summary' of 
hydrographic events useable in the analyses of changes 
in water column productivity (Chapter 4), benthic 
communities (Chapter 5) and vegetation communities 
(Chapter 6), the compiled hydrographic data record 
was divided into separate "events" (or event-duration 
analysis) (Ward 2000). A hydrographic "event" was 
considered to include one or more of the following 
response mechanisms: 1) a substantial volume of 
freshwater flow, 2) an increase in water level (stage) or 
3) a decrease in salinity. Where and how each of these 
three broad response mechanisms were characterized 
depended largely upon the availability' of data. In 
general, the responses of freshwater inflow and water 
level were each divided into two components; one for 
the upper Nueces Bay (and lower delta), and one for 
Rincon Bayou (and upper delta). The response of 
salinity was considered for the hydrographic analysis 
only in upper Nueces Bay, but was examined in each of 
the subsequent chapters at the various monitoring 
stations. The initial conceptual model for such an 
event was a flood hydrograph translating down the 



Three dme scales were considered of pertinence to the 
evaluation of the compiled hydrographic data for the 
Nueces Estuary: intratidal, intertidal and event- 
duration. The intratidal (or intradiumal) time scale 
represented short-term behavior at an hourly 
resolution; the intertidal (or interdiumal) time scale 
represented day-to-day variations of hydrographic 
parameters averaged over 24 hours; and the event- 
duration scale extended over the time period 
encompassing all responses to a specific hydrographic 
event, and could range from several days to many 
weeks. Therefore, to facilitate detailed analysis at 
varying degrees of detail, all available data was 
compiled in hourly, daily and event-specific formats 
(Ward 2000). Further discussion of the approach and 
results of the intratidal, intertidal and event-duration 
analyses conducted by Ward (2000) has been included 
as Appendix B of this Concluding Report. 




Figure 3-3: View of the Rincon gauge 
(Station 08211503). 

Photo courtesy of the Bureau of Reclamation. 



3-4 



Hydrography 



Nueces River into the vicinity of the upper delta. 
However, as became evident, other hydro- 
meteorological processes would also effect similar 
responses in these variables. 

Criteria for Event Delineation 

After inspection of the entire period of record on both 
intertidal and intratidal scales. Ward (2000) formulated 
criteria to identify an event based upon the separate 
hydrographic behaviors of each of the key response 
parameters. These response variables included stage 
(in Nueces Bay, in Rincon Bayou and the super- 
elevation of Rincon over Nueces Bay), flow (in the 
Nueces River and in Rincon Bayou) and salinity (in 
Nueces Bay) (Table 3-2). It should be emphasized that 
these criteria were ultimately arbitrary but were utilized 
to ensure an objective selection of candidate events for 
analysis. Precipitation was not treated as a separate 
hydrographic variable, though it was certainly an 
important hydrographic element in understanding the 
response of the delta ecology. The reason for its 
exclusion as a defining parameter was that it provided 
no in forma tion/)«'rj'(? on the response of the Nueces 
Estuary or Delta. A similar argument was made for the 
exclusion of wind as a defining criterion. 



Once these criteria were established, the daily data for 
the period of study was manually inspected (Ward 
2000). Individual occurrences within the record which 
met at least one of the six criteria were identified as 
hydrographic events. Then, for each event, the 
24-hour mean data for aU hydrographic variables 
during the event were separated and transferred for 
individual analysis. The duration period for each event 
was at least that for which the defining criterion was 
satisfied, though often a longer event period was 
chosen to be sure that the complete response of the 
bay or delta was included in the analysis. When several 
hydrographic events overlapped (i.e., when several 
variables each satisfied criteria separately and 
simultaneously), the event duration was at least the 
period from the first occurrence of the criterion 
threshold for the earliest parameter to at least the last 
such threshold for the latest parameter. 

The greatest difficxilty reported in separating such 
events was met when a time series of events occurred 
in which the response of one parameter overlapped 
that of the next. For example, a series of river 
hydrographs might occur, each of which raised the 
Rincon stage or CalaUen flow above the threshold 
defining an event, and a new surge of inflow occurred 



Table 3-2: Criteria used to define hydrographic events in the data record by response variables. 

Source: Ward 2000. 



Response 
Parameter 



Location 



Defining Criteria of a Hydrographic Event 



Flow 



Stage 



Salinity 



Nueces River A 24-hour mean (daily) flow in the Nueces River at Calallen exceeding 

14.2 mVs (500 cfs). 

Rincon Bayou A 24-hour mean (daily) flow in Rincon Bayou, either positive or negative, 

exceeding 0.28 mVs (10 cfs). 

Nueces Bay A 24-hour mean (daily) stage in the water elevation of Nueces Bay exceeding 

0.30 m (1 .0 ft), referenced to the consistent CBI datum from Ward (1997), 
established by "empirical leveling". 

Rincon Bayou A 24-hour mean (daily) stage in the water elevation of Rincon Bayou exceeding 

0.61 m (2.0 ft), relative to Rincon gauge datum, which is 422 cm above the 
consistent datum for CBI gauges. 

Super-elevation The difference of Rincon Bayou minus Nueces Bay daily stage values 
exceeding 0.15 m (0.5 ft), referenced to common datum. 

Nueces Bay Change in salinity concentrations of Nueces Bay exceeding 5 ppt over a five 

day period. 



Chapter Three ♦ 3-5 



before the recession of the preceding event has 
subsided. Separating these into individual events was 
therefore rather arbitrary, and, from the estuarine 
response point of view, this division may or may not 
have been differentiated in the actual environment, 
which may have responded as if adjacent events in the 
record were a single "merged" event. 

Characterization of Individual Events 

Once individual events were thus delineated in the 
record, integrated values for each response parameter 
in each event were computed. The term "integrated" 
means either averaged or accumulated, whichever was 
more meaningful for the parameter under considera- 
tion. The parameters compiled for each event include 
the following: event number, date, duration, rainfall, 
flow, stage and salinity. 

Event Number — For purposes of tracking and 
reference, each identified event occurring from 
October 1, 1994, through December 31, 1999 (the 
duration of the monitoring program), was numbered 
sequentially in time. 

Date and Duration — The span of each event was 
specified by its starting date and its duration in days. 
In some cases, the ending date of one event was 
immediately before the starting day of the next, which 
indicates that a subjective separation had been assumed 
in the record for purposes of analysis. 

Rainfall — Local precipitation for each event was 
determined by summation of daily totals during the 
event. For the period of October 1, 1994, through 
May 15, 1996 (i.e., prior to the activation of the Rincon 
gauge), precipitation was that reported at Corpus 
Christi Airport, which e\'idenced a correlation with the 
Rincon gauge of 0.81 for the 3.7 years of coincident 
data. After May 15, precipitation was that reported by 
the Rincon gauge. 

Flow — Daily values for both flow variables (Nueces 
River and Rincon Bayou) were converted to daily 
volume and then summed to determine the cumulative 
event volume. For flow in the Nueces River, daily 
average flow was also computed for each event by 
dividing the total flow volume by the event duration in 



days. Because flow in Rincon Bayou was frequendy bi- 
directional, these data were analy2ed somewhat 
differendy. Total positive volume (pos) was 
determined by considering only that flow directed from 
the river into the delta. Total negative volume (neg) 
was similarly determined but considering only that flow 
directed from the delta into the river. Total exchange 
(gross) was defined to be the sum of the absolute 
values {i.e., ignoring signs) of the positive and negative 
volumes. Finally, total net exchange (net) was defined 
to be the algebraic sum {i.e., observing signs) of the 
positive and negative volumes. 

It is important to note that, during the demonstration 
period, the Nueces River did not exceed the natural 
flooding threshold for the delta, which was 1.64 m 
(5.40 ft) (Bureau of Reclamation 2000), except on four 
occasions. This means that, except for these events 
(Events 16, 18, 25 and 36), the only water exchanged 
between the Nueces River and Rincon Bayou during 
the study period passed through the Nueces Overflow 
Channel. During the excepted events, an additional 
amount of water also entered Rincon Bayou naturally 
\'ia the low depressions along the bank of the river. 
Therefore, for all other events, the Rincon Bayou flow 
volumes reported were those measured through the 
Nueces Overflow Channel at the Rincon gauge. For 
Events 16, 18, 25 and 36, an additional volume was 
added to that gauged through the overflow channel. 
This additional amount depended upon the daily stage 
level attained by the river during the event and was 
estimated using the hydraulic model developed by 
Reclamation (2000). Accordingly, for Event 16, which 
attained a peak daily stage of 1.70 m (5.57 ft) msl, an 
additional 4 10^ m' (3 acre-ft) were added; for Event 
18, which attained a peak daily stage of 1.72 m (5.65 ft) 
msl, an additional 5 10^ m' (4 acre-ft) were added; for 
Event 25, which attained a peak daily stage of 2.22 m 
(7.28 ft) msl, an additional 1,189 10' m' (964 acre-ft) 
were added; and for Event 36, which attained a peak 
daily stage of 1.74 m (5.72 ft) msl, an additional 6 10' 
m' (5 acre-ft) were added. The remark "natural flow 
event" identifies these events in Table 3-3. 

Stage — For the water level in Nueces Bay and Rincon 
Bayou, both the daily average and peak daily stages 
were utilized, the latter since the average alone might 



3-6 ^* Hydrography 



not have fairly depicted the range of water level 
excursion during an event. Raw daily stage data from 
the Rincon gauge, which is reported in local time, was 
corrected for GMT {i.e., the 15-minute data were re- 
averaged) to allow temporal consistency with the other 
data variables. The super-elevation of water levels, 
determined by subtracting the Nueces Bay stage from 
the Rincon Bayou value, was used to determine the 
predominant influence on stage in upper Rincon 
Bayou. On an instantaneous basis, the super-elevation 
is the direct force for discharge through the Nueces 
Overflow Channel. 

Salinity — The salinity response of upper Nueces Bay 
to an inflow event would be expected to be an initial 
drop in concentration, followed by a slow increase with 
time, or "recovery". However, this approach of 
integrating the salinity parameter could possibly 
understate the response of salinity to lower magnitude 
hydrographic events, because the entire response could 
be occur within the event's duration and therefore 
elude detection. Foxir separate indicators were thus 
computed from the salinity data for each hydrographic 
event: 1) the salinity value at the beginning of the event 
(start), 2) its net incremental change at the close of the 
event (chg), 3) the percent of the beginning value that 
the increment represented (% chg), and 4) the 
minimum value attained during the event (min). As so 
defined, the sign of the incremental (percent) change 
indicates whether salinity increased or decreased over 
the duration of the event (negative for a decrease and 
positive for an increase). 



RESULTS 

In this analysis, hydrographic events were delineated 
according to the mechanism or feature exhibiting a 
response: namely, the response of flow in the Nueces 
River or in Rincon Bayou; water level in Nueces Bay or 
in Rincon Bayou; and salinity in upper Nueces Bay. A 
oirsory inspection of the hydrographic events 
presented in Table 3-3 reveals the fact that each of 
these response mechanisms can occur in isolation, 
without the involvement of the others. This indicates 
that a certain degree of care must be used in 
determining how a hydrographic event has influenced 



the chemistry and ecology of the project area. For 
example, if inundation and de-watering were of 
concern, then the extent to which events accomplished 
a response of water level would be of central interest, 
and the most important of these was the response of 
water level in Nueces Bay for the lower delta and in 
Rincon Bayou for the upper delta. If freshwater inflow 
to Nueces Bay from the river were the major 
determinant, perhaps through a sediment or chemical 
load, then the flow events in the Nueces River would 
be regarded as most important. If it were freshwater 
flow into the upper delta, then flow in Rincon Bayou 
would be critical. Finally, if the depression of salinity 
were a key interest, then the salinity events in Nueces 
Bay would be of primary concern. 



Overview of Hydrographic Events 
That Occurred During the 
Demonstration Period 

For the period investigated {i.e., October 1994 through 
December 1999), a total of 37 hydrographic events 
were identified (Table 3-3). Five of these events 
occurred prior to the opening of the Nueces Overflow 
Channel, and thirty-two afterward. These events were 
highly variable in the magnitude of their responses, 
durations and in the subset of hydrographic variables 
in which a response occurred. Some were associated 
with seasonal high waters in Nueces Bay, some were 
salinity responses elicited only by internal circulations 
of the bay, some were responses to intense rainfall and, 
of course, some were in fact inflow hydrographs in the 
Nueces River. Most (28) of these events occurred after 
operation of the USGS Rincon gauge in the Nueces 
Overflow Channel, allowing the direct measurement of 
flow diverted by the demonstration project, the 
associated water level rise and in situ precipitation. 

Of the total 37 events observed, 1 5 met the flow 
criteria for a flow event in the Nueces River, 16 met 
the criteria for a stage event in Nueces Bay and 21 
met the criteria for a salinity event. Of the 28 events 
occurring after the installation of the Rincon gauge, 
20 met the criteria for a flow event in Rincon Bayou, 
27 met the criteria for a stage event in Rjncon Bayou 
and 14 met the criteria for a super-elevation event. 



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3-8 ♦♦♦ Hydroff-aphy 



Water level in Rincon Bayou (at least since the Nueces 
Overflow Channel was opened) was the most 
responsive parameter to hydro-meteorological forces in 
the upper delta during the demonstration period 
(Table 3-3). 



Summary of Selected Individual 

Events 

As can be observed from Figures 3-4 through 3-9, 
most of the observed hydrographic events involving 
large flows in the Nueces River occurred during the 
latter portion of the demonstration period, particularly 
during 1997, 1998 and 1999. This absence of such 
events in the early part of the demonstration period 
was attributable to the fact that, during the first few 
years, south Texas experienced a moderate to severe 
drought. However, although large freshwater diversion 
events were absent during this period, modest flow 
events into Rincon Bayou from the Nueces River did 
occur {e.^.. Events 12, 13 and 14, and probably a few of 
Events 6 through 10). The driving mechanism for 
these events was not flow in the river but other hydro- 
meteorological forces affecting water level in the 
Nueces Estuary (Table 3-3). Several other similar 
exchange events occurred throughout the 
demonstration period. 

Fall 1996: Events 12 through 14 

One example of these kinds of small exchange events 
were Events 12, 13 and 14, which resulted from a fall 
maxima high water event in the Gulf of Mexico 
(Table 3-3). From October 3 through 8, as the water 
levels in Nueces Bay and Rincon Bayou increased, a 
sustained positive flow occurred through die overflow 
channel, which peaked on October 6 at 1.04 m'/s 
(36.9 cfs) (Figure 3-10). During same period, only a 
minimal amount of river flow (no more than 
0.10 m^/s, or 3.5 cfs) passed over Calallen Diversion 
Dam. However, during October 9 through 14, flow in 
the river rose to about 4.67 m'/s (165 cfs) on 
October 12, while water levels in the upper delta and 
bay began to decrease. The surprising result was that 
water diversion through the channel reversed direction 
and flowed from the upper delta into the Nueces River 



until October 15, when the water level in the bay again 
began to increase. This behavior, which continued 
through Events 13 and 14, demonstrated that, at low 
flow volumes in the Nueces River, diversions through 
the overflow channel were driven primarily by water 
level variations in Nueces Bay and the upper delta. 

The USGS made several salinity measurements at the 
Rincon gauge during Event 12. Salinity was measured 
on October 4, 5 and 6, which was the period of 
sustained positive flow through the overflow channel. 
The salinity values for each day were 2.0, 3.9 and 
7.2 practical saHnity units (psu) (which are approxi- 
mately equivalent to parts per thousand), respectively. 
During this period, no flow occurred in the Nueces 
River on October 4 and 6, and only 0.10 m'/s (3.5 cfs) 
occurred on October 5. The increasing salinity values 
over this 3-day period indicates that flow was moving 
up the Nueces River channel and into Rincon Bayou as 
a result of the rising water level in Nueces Bay. 
Therefore, the total net diversion into Rincon Bayou 
during these three events (289 10' m', or 234 acre-ft) 
was relatively fresh water. 

Summer 1997: Events 16 and 17 

The first significant occurrence of freshwater flow 
during the demonstration period occurred from June 
21 through July 27, 1997 (Events 16 and 17). These 
two events occurred one immediately after the other, 
and were derived from the same basin-wide 
precipitation event. This storm was one of the many 
tropical/middle latitude heavy rain events common to 
south Texas. On June 21, a near-stationary low 
pressure system over south-central Texas began to 
move east and north, causing scattered showers and 
thundershowers over a large part of north Texas, the 
Texas Hill Country and central Texas. This movement 
allowed tropical moisture to move in from the south 
and feed into the area of instability, lift and daytime 
heating in the afternoon, which resulted in a second 
round of locally heavy rain in the greater Nueces 
watershed. 

Rainfall amounts with this second rain event varied 
from 23 to 58 centimeters (cm) (9 to 23 inches) over 
the Texas Hill Country, and between 13 and 25 cm 



Chapter Thne ♦ 3-9 



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Chapter Tine ♦ 3-15 



2.25 




1 r 

10 12 14 16 18 20 22 24 26 28 



30 1 3 
Nov 



Flow in Rincon Bayou (Rincon gauge) 



Flow in Nueces River (Calallen gauge) 

— o— Water level in Nueces Bay (White Point gauge) 



Figure 3-10: Selected hydrographic data for Events 12, 13 and 14 (October 2 through 
November 3, 1996). During this period of low flow in the Nueces River, water level in Nueces Bay 
(not flow in the Nueces River) was the predominant factor in determining the rate and direction of 
flow through the Nueces Overflow Channel and Rincon Bayou. 



(5 to 10 inches) over central and south-central Texas, 
including the headwaters of the Nueces and the Frio 
drainages. A similar pattern, on a larger scale, affected 
central and south central Texas in December of 1991, 
causing 30 to 51 cm (12 to 20 inches) of precipitation 
over much of south Texas. Direct precipitation in the 
upper delta from the 1997 storm totaled only 4.9 cm 
(1.94 inches) through June 21-23, -with the only other 
measurable precipitation being 0.1 cm (0.04 inches) 
fromjuly20to21. 

These two hydrographic events (16 and 17) 
exemplified a common resulting flow pattern in the 
lower Nueces River from precipitation events in the 
greater watershed: namely, a duel-peak hydrograph in 
the river at the point of diversion into the delta 



(Figure 3-7). The first principal peak (Event 16) 
arrived at the upper delta on June 26, and represented 
runoff primarily in the lower (eastern) watershed. 
Approximately 20 days later, during which time the 
Nueces River at Calallen experienced a sustained flow 
of about 39.6 m'/s (1,400 cfs), the second principal 
peak (Event 17) arrived on July 15. This peak 
represented runoff from the same storm but from the 
upper (western) watershed. As a result of the total 
river inflow into the bay system, the salinity of Nueces 
Bay was reduced from 26.8 ppt at the beginning of 
Event 16, to 3.4 ppt at the end of Event 17. 

Because the average ambient water level in Nueces Bay 
was so low and the average stage in the river at the 
point of diversion was so high, the hydraulic head 



3-16 ♦♦♦ Hydrography 



imposed by the river (as represented by mean super- 
elevation) was considerable (greater than 1.21 m, or 
4.0 ft) (Table 3-3). This indicates, unlike during Events 
12 through 14, the diversion rate into Rincon Bayou 
during Events 16 and 17 was predominandy a fiinction 
of the elevated stage in the Nueces River. As each 
principal peak receded, discharge through the overflow 
channel dropped sharply (Figure 3-7). At the event's 
conclusion, the loss of stage in the river immediately 
resulted in a sharp decrease in the rate of discharge, 
and the upper delta discharged a significant amount of 
water back into the river. This pattern, the reversal of 
diverted flow at the end of large river flow events, 
would become common in subsequent events. During 
both events, the Rincon Overflow Chaimel was 
activated, diverting water into the extensive tidal flat 
area of the upper delta. 

FaU1997: Event 18 

The fall event of 1997 (Event 18) resulted from very 
heavy precipitation in the lower Nueces Watershed. 
This event, although comparable to Event 16 in 
regards to positive flow and maximum stage in Rincon 
Bayou, was different from the previous two events in 
one significant way. Unlike Events 16 and 17, which 
were events responding to precipitation higher in the 
basin. Event 1 8 was a hydrographic response to 
intense local precipitation direcfly on the study area 
and lower watershed. Approximately 23 cm 
(9.17 inches) fell within the area during the first 6 days 
of the event, contributing to heavy local runoff and 
artificially elevated water levels in the upper delta 
and Nueces Bay (as reflected in mean and peak stage). 
As a result, discharge into the delta from the river 
actually began before the river (gauged at Calallen) 
began to respond to the event (Figure 3-7). 

As with the two previous events, the rate of diversion 
into the delta fell off sharply as soon as stage in the 
river began to drop, and, like Event 17, remained 
negative for the concluding days of the event. 
Although the total positive flow diverted into Rincon 
Bayou during this event was about the same as that for 
Event 16, over 60% of this volume flowed back into 
the river at the end of the event. This difference was 
attributable to the fact that water levels in the Nueces 



River at the point of diversion (and therefore, the 
hydravilic head) were not maintained after the river 
crested, as was the case with Event 16. 

FaU 1998: Events 21 through 27 

For purposes of event analysis, the fall of 1998 was a 
very complicated period. The careful observer of 
Table 3-3 and Figure 3-8 will recognize that, as with 
Events 12 through 14, Events 21 through 27 were 
essentially contiguous, or that the ending day of each 
event immediately preceded the beginning day of the 
next. The challenge in interpretation was separating 
the effects of a bewildering number of influencing 
factors. For example, during this 96-day period, two 
tropical storms made landfall in the region, a fall- 
maxima high water event occurred in the Gulf of 
Mexico, over 56 cm (22 inches) of local rainfall was 
recorded in the study area, over 219,820 10' m' 
(178,194 acre- ft) flowed from Nueces River into the 
bay and over 5,092 10' m' (4,128 acre- ft) was diverted 
into the upper delta. For purposes of interpreting 
biological responses in the delta, one may justifiably 
consider each numbered event as a mere temporal 
component of the greater autumn occurrence of 1998. 

Beginning on August 17, heavy rain episodes caused by 
a cool air mass sagging into central Texas began to 
occur over much of south Texas. Because the rainfall 
followed record drought conditions between April and 
July, very httie runoff resulted. In fact, over 15 cm 
(6 inches) of precipitation feU over a 2,000-km" area in 
the Frio River watershed and produced absolutely no 
flooding (Patton 1998). The rainfall continued 
throughthe day on August 1 8, with a wide-spread area 
in the greater Nueces watershed receiving 20 to 33 cm 
(8 to 13 inches). 

Tropical Storm Charley made landfall near Port 
Aransas the night of August 21 (Events 21 and 22). 
As the storm center moved slowly inland toward the 
Hill Country west of San Antonio, it produced 5 to 
8 cm (2 to 3 inches) of rain in a three-hour period, 
with the heaviest rainfall resulting from "feeder" bands 
that wrapped around the center. These bands were 
moving very slowly and dropped several inches of rain 



Chapter Thne ♦ 3-17 



before they departed an area. Primarily because the 
region had received large rainfall totals for several days 
prior, significant and fatal flooding on the upper 
Frio and Nueces rivers resxilted. Direct precipitation 
in the delta from the storm's landfall totaled only 
about 0.6 cm (0.24 inches) (August 21 to 23). 
Nevertheless, a modest peak in stage and discharge in 
Rincon Bayou occurred on August 23 and 24, primarily 
due to the storm surge (Figure 3-8). Because a 
relatively small amount of precipitation fell in the lower 
Nueces watershed associated with the landfall of 
Charley, the Nueces River at Calallen recorded only 
a modest peak on August 8. 

However, as previously mentioned, Charley did result 
in a broad flooding event in the western basin. The 
Nueces River at Calallen began to respond to this 
flooding on September 4, and crested on September 14 
(Event 23). Flow through the Nueces Overflow 
Channel generally followed that of the river, but 
fluctuated on a hourly and daily basis. This oscillation 



was primarily due to the complicating effect of a 
second storm surge associated with Tropical Storm 
Frances, which made landfall near San Antonio Bay on 
September 10. The arrival of Frances may be observed 
in Figure 3-11, when, during September 7 through 10, 
stage and flow in Rincon Bayou increased dramatically, 
while stage in the Nueces River remained essentially 
constant. Similarly, when the surge subsided after 
September 10, discharge through the overflow channel 
dropped sharply, even though flow in the Nueces River 
at Calallen increased significantiy. Direct precipitation 
in the delta from Tropical Storm Frances totaled 
5.7 cm (2.26 inches) from September 9 to 12. 
Flow in the Nueces River continued to remain above 
19.8 mVs (700 cfs) (Events 23 and 24) for several 
weeks after the landfall of Frances as a result of the 
storm's heavy rain in the upper watershed. Flow in 
Rincon Bayou was also generally positive during this 
period but not substantial (less than 0.28 m'/s, or 
10 cfs). As with the previous events, flow in Rincon 



300 



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Flow in Rincon Bayou (Rincon gauge) 

Flow in Nueces River in 10's of cfs (Calallen gauge) 

Water level in Nueces Bay (White Point gauge) 



Figure 3-11: Selected hydrographic data for Event 23 (September 1 through 31, 1998). 



3-18 ♦ Hydrography 



Bayou became negative immediately after stage in the 
river began to receded beginning October 13 
(Figure 3-8). 

Due to significant runoff from the large amounts of 
tropical moisture and associated precipitation in the 
upper Nueces Basin during this period, the Nueces 
River again began to rise, cresting on October 21 
(Event 25). This crest resulted in an estimated daily 
flow rate of 73.6 m'/s (2,600 cfs) in the Nueces River 
at Calallen, the highest flow rate attained during the 
demonstration period. The corresponding water 
elevation in Rincon Bayou was also the highest daily 
stage recorded during the study, 2.22 m (7.28 ft msl), 
which was 0.48 m (1.56 ft) higher than the next highest 
event daily peak (Event 36). Because of this. Event 25 
was the only event to witness the Nueces River 
meaningfully exceed the natural {i.e., without-project) 
diversion threshold of the upper Nueces Delta, which 
was 1.64 m (5.40 ft) msl. As a result, the estimated 
amount of delta inflow over the natural river bank 
(1,204 10' m\ or 976 acre-ft) was about 50% as much 
as that which was gauged through the overflow 
channel (2,560 10' m', or 2,075 acre-ft). It was during 
this event that the road crossing at the lower end of the 
Rincon Overflow Channel washed out (Figure 3-12). 

Events 26 and 27 concluded the fall occurrence of 
1998, resulting in two more smaller peaks in river and 
diversion flows as the greater watershed finally drained 
the accumulated runoff of the preceding months. 

FaU1999: Events 35 and 36 

Hurricane Bret was the first major hurricane of the 
Adantic 1999 season (Figure 3-13). The storm formed 
in the southern Gulf of Mexico and moved slowly 
northward along the Mexican Coast. As it approached 
south Texas, it rapidly intensified into a Category 4 
hurricane. Landfall was made as a Category 3 storm on 
August 23, in a remote area between Brownsville and 
Corpus Christi (Event 35). The storm surge from Bret 
was substantial, and combined with the 14.43 cm 
(5.68 inches) of local precipitation that fell on August 
22 and 23, the Rincon gauge recorded a maximum 
elevation of 1.76 m (5.79 ft) msl (August 23). This 
value was the highest value recorded during the study 




Figure 3-12: Discharge into the tidal flats area through 
the road crossing at the north end of the Rincon 
Overflow Channel during Event 16 (June 27, 1997). 

Each of the 10 corrugated HOPE culverts shown were 24"- 
diameter. This structure was subsequently washed out 
during Event 25 (October 1998). 

Photo courtesy of the Bureau of Reclamation. 



period without a corresponding flow ev^ent in the 
Nueces River. Once the low pressure system had 
moved on-shore, the surge tide and local runoff in the 
upper delta began to drain out, resulting in a net 
negative flow through the overflow channel for Event 
35 (Figure 3-9). 

As a result of heavy rainfall in the watershed associated 
with Hurricane Bret, particularly in the southwestern 
portion, the Nueces River again responded with a large 
hydrograph beginning on September 5 (Event 36). 
The amount of diverted flow through the overflow 
channel was the second largest recorded for any event, 
with a net flow of 1,052 10' m' (853 acre-ft) over 
17 days. The salinity of Nueces Bay, which had already 
been lowered by the landfall of Hurricane Bret some 
three weeks before, continued to decrease as a result of 
the event. 



Chapter Thm ♦ 3-19 




Figure 3-13: Visual satellite image showing the 
location of Hurricane Bret in relation to the Nueces 
Watershed just before landfall (August 22, 1999). 

Having threatened to lay waste to much of south Texas, 
the storm made landfall as a Category 3 Hurricane. 
Meteorologists were surprised, however, by Bret's 
lackluster performance inland. "We ain't surprised," said 
one Texas native. "You think something named 'Bret' is 
gonna do much here? I have breakfast cereal with more 
hair on its chest." 

Source of Image: National Environmental Satellite, Data and 
Information Service of the National Oceanic and Atmosptieric 
Administration. 



DISCUSSION 

Flooding Thresholds for the Upper 
Nueces Delta 

For over 1 3 years prior to the opening of the Nueces 
Overflow Channel {i.e., October 26, 1995), the 
minimum flooding threshold required for the Nueces 
River to spiU freshwater into the upper Nueces Delta 
was rarely attained (Irlbeck and Ward 2000). Based 
upon recent estimates, a flow in the Nueces River at 
Calallen greater than about 59.5 m'/s (2,100 cfs) was 
required to breach the lowest portion of the northern 
river bank (Irlbeck and Ockerman 2000). However, 
excavation of the overflow channel fundamentally 
changed this condition. The minimum flooding 
threshold was thus lowered from 1.64 m (5.4 ft msl) to 
about 0.0 m msl. As can be observed from the 
suinmary of hydrographic events in Table 3-3, this 
change not only allowed more frequent diversions of 



fresh water into Rincon Bayou and the upper delta, but 
also provided the opportunity for other non-riverine 
elements like wind and tide to force water exchanges 
between the upper delta and the Nueces River. As a 
result, near continual {i.e., daily) exchange between the 
Nueces River and the upper delta was observed during 
the demonstration period, whereas before the 
interaction was limited to only extremely rare river 
inflow events. 

During the 50-month period when the overflow 
channels were open, over 8,810 10' m' (7,142 acre-ft) 
was diverted from the Nueces River into Rincon 
Bayou and the upper delta. Using the hydraulic model 
developed by the Bureau of Reclamation (2000), it was 
estimated that, of this total amount, only about 1,204 
lO' m' (976 acre-ft) would have been diverted without 
the demonstration project features. Therefore, during 
the demonstration period, the total volume of 
freshwater inflow into the upper Nueces Delta was 
increased by about 732% from what would have 
occurred without the project. In a longer-term 
examination, Irlbeck and Ward (2000) analyzed the 
inflow patterns of the upper delta assuming that 
demonstration project features had been in place since 
the completion of Choke Canyon Dam in 1982 
through 1999. These authors concluded that the 
average annual inflow amount to the upper delta 
during this 17.6-year period would have been increased 
from about 666 10' m' (540 acre-ft) to approximately 
4,219 10' m' (3,420 acre-ft), or by over 633% from that 
which would have occurred without the project 
(Irlbeck and Ward 2000). This long-term analysis has 
been included as Appendix C of this Concluding 
Report. 



Activation and Behavior of 
Demonstration Project Features 

Nueces Overflow Channel 

Because the controlling (bottom) elevation of the 
Nueces Overflow Charmel was at or near mean sea 
level, it was activated (or it allowed the exchange of 
water) when there was a change in water level in either 
bay or the river. Throughout most of the 



3-20 V Hydrog'aphy 



demonstration period, bi-directional flow occurred 
almost daily. Changes in the physical condition of the 
Nueces Overflow Channel included the encroachment 
of vegetation along the water's edge, and a slight 
narrowing of the bottom of the channel due to erosion 
from its banks (Figure 3-14). 

Since the primary purpose of the demonstration 
project was to divert a portion of the flow in the 
Nueces River through the diversion channel, a logical 
inquiry was the proportion of such a flood so diverted. 
Upon examination of the relation between the total 
event flow volume in the Nueces River and in the 
Nueces Overflow Channel, it was determined that the 
volume diverted into Rincon Bayou increased generally 




Figure 3-14: The Nueces Overflow Channel ioo,„.i9 
southwest. Note the changes in vegetation and channel 
characteristics from October 1995, immediately after 
construction (above), to June 1999, towards the end of the 
demonstration period (below). 

Photos courtesy of the Bureau of Reclamation. 



with the flow in the river, and the actual proportion of 
the flow amount diverted was on the order of 2% of 
that in the river (Ward 2000). The actual rate of 
discharge, however, varied considerably between 
events depending upon the water level in Nueces Bay 
and Rincon Bayou. 

That the relation between Nueces River event volume 
and the volume transported through Rincon Bayou 
should depend upon water level was not unexpected, 
based upon hydraulic considerations. Unlike a river 
channel system in which the head gradient and the 
water level (stage) are closely related, there is no direct 
relation between water level and flow in the Nueces 
River below Calallen Diversion Dam because of the 
corrupting effect of tidal and meteorological 
water-level variations. For the events observed, the 
Nueces River hydraulic head was superposed on 
whatever water level was present in Nueces Bay, which 
affected how the river head could drive flow through 
the overflow channel. Deeper water made available a 
greater cross-section area of the channel and lowered 
the frictional resistance. Therefore, a given hydraulic 
head in the Nueces River drove a greater flow through 
the diversion channel when the Nueces Bay water level 
was higher. 

Rincon Overflow Channel 

In addition to increased inflow, the demonstration 
project features also increased the distribution of 
diverted fresh water within the tidal flats of the upper 
delta. The controlling elevation of the Rincon 
Overflow Channel was about 1.14 m (3.75 ft) msl. 
When water levels in Rincon Bayou exceeded this 
threshold, flow would pass through the chaimel and 
across the tidal flats to the northeast. Without this 
overflow channel, total diversions through the 
demonstration project would have been lower, and 
most of the freshwater diverted would have remained 
channelized in the upper delta. 

Although no direct gauging data were available to 
determine exactiy when and to what degree the Rincon 
Overflow Channel was activated during the 
demonstration period, it is certain that, on at least two 
occasions, the channel passed a significant amount of 



Ch^ter Three ♦ 3-21 



freshwater into the tidal flats. The first ocairrence was 
during Event 16, when such discharge was observed in 
the field, and the second was during Event 25, when 
the road structure at the north end of the channel was 
washed out into the tidal flats (Figure 3-15). Based 
upon comparable event stages, super-elevations and 
flow volumes, it was strongly suspected that the 
Rincon Overflow Channel also passed some amount of 
diverted fresh water during Events 17, 18 and 36, 
although this was not visually verified. 




Figure 3-15: The Rincon Overflow Channel. The road 
crossing structure, as shown from the air during Event 16 
in June 1997 (above), w/as washed out during Event 25 in 
October 1998, as shown in June 1999 (below). The 
HOPE culverts (foreground of the lower photo) were 
24"-diameter, and the livestock (background of the 
lower photo) were Red Brangus. 

Ptiotos courtesy of the Bureau of Reclamation. 




Figure 3-16: The low water crossing at the head of the 
upper Rincon Bayou channel. The integrity of this 
structure, although significantly over-topped during Event 
25 (October 1998), remained unchanged throughout the 
demonstration period. Each HOPE culvert was 
36"-diameter. The photo was taken on June 1999. 

Photos courtesy of thie Bureau of Reclamation. 



Low Water Crossing 

The low water crossing at the head of Rincon Bayou 
performed exceptionally well. All flows diverted from 
the river (either naturally or through the Nueces 
Overflow Channel) passed through this structure 
(Figure 3-16). On at least one occasion (Event 25), a 
significant amount of water passed over the top of the 
crossing, with no damage visible. At the end of the 
demonstration period, the structure was essentially the 
same as at the beginning. 



SUMMARY 

The demonstration project features significandy 
lowered the minimum flooding threshold of the upper 
Nueces Delta, and thereby increased the opportunity 
for larger, more frequent hydrographic events. Based 
on observations and data analysis during the 
demonstration project, such events were categorized 
into three general types: small "exchange" events, 
"positive-flow" events and "tidal flat inundation" 
events. 



3-22 ^* Hydmgraphy 



Exchange Events 

Exchange events were considered to be frequent, low- 
volume interactions between the channels and pools of 
Rincon Bayou and either adjacent water body (Nueces 
Bay or Nueces River) (Figure 3-17). Exchange events 
were primarily caused by daily differences in water level 
elevations, although these differences were in turn the 
result of a variety of other factors like tide, wind, river 
inflow, etc. The net flow volume for these types of 
events was generally low (less than 123 10' m', or 
100 acre- ft), and could be either positive or negative 
through the Nueces Overflow Channel. Although the 
effects of exchange events were confined to the 
channels of Rincon Bayou, they provided considerable 
dilution and mixing of ambient waters, especially in the 
upper delta. Events 11, 15, 22 and 31 were typical 
examples of exchange events (Table 3-3). Prior to the 
demonstration project, the Rincon Bayou and the 
upper delta were completely isolated from such daQy 
interactions with the river. 



Positive-Flow Events 




Figure 3-17: Typicai v:i.. of the Nueces Overflow 
Channel during tidal exchange. The view is looking 
downstream (east) from the Nueces River. 

Photos courtesy of the Bureau of Reclamation. 



the demonstration period, the amount of fresh water 
diverted into the upper Nueces Delta was increased by 
about 732%, and most of this increase was attributable 
to positive-flow events. 



The second event type, positive-flow events, were 
considered to be infrequent, large-volume events which 
resulted in a positive flow of water from the Nueces 
River into Rincon Bayou (Figure 3-18). Because these 
events were primarily driven by flow events in the 
Nueces River, they typically occurred during the spring 
or faU. Unlike exchange events, the volumes associated 
with positive-flow events (usually greater than 
123 10' m', or 100 acre-ft) did not simply dilute water 
in Rincon Bayou but also displaced it to a considerable 
extent. Because of their magnitude, positive-flow 
events were not confined to Rincon Bayou but 
frequendy affected the lower adjacent flats, channels 
and pools. Depending upon their magnitude, such 
events might or might not have also inundated the 
higher marshes and tidal flats of the delta. As 
previously discussed, the actual diverted volume and 
effectual extent of any one such event was greatiy 
dependant upon the ambient water level in Nueces 
Bay. Events 26, 27, 29 and 34 were typical 
examples of positive-flow events that did not inxmdate 
higher adjacent marshes and flats (Table 3-3). During 




Figure 3-18: View of the upper Rincon Bayou 
(background) during a typical positive-flow event 
(Event 16). The head-water channel of Rincon Bayou 
(about 300 m downstream of the Nueces Overflow 
Channel) is in the foreground, and the western-most 
MoPac Railroad bridge is center. Upper Rincon Bayou is 
in the background. The photograph was taken on 
June 26, 1997. 

Photo courtesy of the Bureau of Reclamation. 



ChapterThree ♦ 3-23 



Tidal Flat Inundation Events 

Tidal flat inundation events were considered to be 
large, positive- flow events during which the Rincon 
Overflow Channel was activated, diverting fresh water 
into the tidal flats of the upper delta and immersing, to 
some degree, those higher marshes (Figure 3-19). 
These events (Events 16 and 25 confirmed, and 
Events 17, 18 and 36 strongly suspected) were 
relatively rare during the demonstration period. 
Although these tidal flats were also periodically 
inundated by other hydro-meteorological forces {e.g., 
the storm surge of Hurricane Bret during Event 35), 
such non-riverine events were not considered to be 
tidal flat inundation events because fresh water was not 
significandy involved in the mechanism. Without the 
demonstration project, these tidal flats would not have 
been direcdy freshened, as the largest of the natural 
diversions that would have occurred (Event 25) would 
not have exceeded the confines of the Rincon Bayou 
channel in the upper delta. 



V 'l^^^^^^^^^^^^^^^^l 


^^■■■■PP 


^ "^•jy^^-Vgj^^B^^B^B 


JUJPH^g 


MV 


'^^^^smHi 


B 


n 



Figure 3-19: View of diverted fresh water in the tidal 
flats area in the upper Nueces Delta during activation 
of the Rincon Overflow Channel (Event 16). The view is 
looking east from the outfall of the overflow channel. The 
photograph was taken on June 27, 1997. 

Photo courtesy of the Bureau of Reclamation. 



3-24 ♦♦♦ Hydrography 



TERRY E. WHITLEDGE 

Institxite of Marine Science, 
University of Alaska Fairbanks 

DEAN A. STOCKWELL 

Institute of Marine Science, 
University of Alaska Fairbanks 



CHAPTER FOUR 

Water Column 
Productivity 



"In water all hath had its primal source; 
and water still keeps all things in their 
course." 

♦ Johann Wolfgang von Goethe (1749-1832) 



INTRODUCTION 

Observations of nutrient and primary productivity 
changes in aquatic environments have been used to 
assess many aquatic ecosystems for changes of primary 
inputs or suspected ecosystem alterations (Boynton et 
al. 1982; Pennock et al. 1999). Estuarine areas, such as 
the Nueces Delta, depend upon the mixing of fresh 
water with sea water to maintain biological 
productivity. Specifically, fresh water imports nutrients 
and dilutes salinity of the receiving sea water. 
Therefore, increased freshwater inflow from the 
demonstration project should have a large impact on 
the water column and its biological processes. The re- 
introduction of fresh water from the Nueces River into 
the upper Nueces Delta offered an opportunity to 
monitor nutrient and primary productivity responses in 
a historic river delta that had been altered by lack of 
freshwater inflows from small and medium runoff 
events. In the recent past, fresh water flow has been 
limited to only large events that have flooded the delta 
every several years. 



OBJECTIVES 

1) To assess the effect of the demonstration project 
on salinity and nutrient availability to the water 
column of the study area; 

2) To assess the response of water column 
phytoplankton populations to changes in salimty 
and nutrient availability; and 



Chapter Four ♦ 4-1 



3) To assess the response of the sedimented 

phytoplankton (microphj'tobenthos) populations 
to changes in salinity and nutrient availability. 



METHODS AND APPROACH 
Study Design 

Water column processes were examined at eight 
sampUng sites located throughout the upper Nueces 
Delta (Figure 4-1). At each sampling station, 
hydrographic data, water samples and mud samples 
were collected to measure temperature, salinity, 



inorganic nutrient concentrations, phytoplankton 
pigments and plankton growth (productivity) rates. 
Samples were collected monthly at most sites during 
the demonstration period unless extreme flooding 
prevented access to the stations or they were dry. A 
total of 493 samples were taken for most of the 
measurements. TTie eight sampling stations were 
chosen to represent the various segments within the 
project area, including one reference site and three 
treatment sites (upper Rincon Bayou, central Rincon 
Bayou and Tidal Flats). One other station (68) in the 
Nueces River near the Nueces Overflow Charmel was 
used to provide a characterization of the riverine 
source. 




Northern 
bluff line 



Like 



Rincon Bayou 



central Rincon Bayou Site 
(treatment) 



Figure 4-1 : Location of water column sampling stations in the upper Nueces Delta. The four sites included one 
reference site (Stations 62 and 63) and three treatnnent sites: upper Rincon Bayou (Stations 65 and 66), central Rincon 
Bayou (Stations 60 and 61) and Tidal Flats (Station 62). In addition to these, one sampling station (Station 68) was 
occupied in the Nueces River near the Nueces Overflow Channel. 



4-2 



Water Column Pmdudivity 



Because phytoplankton are relatively short-lived and 
are mobile within the water column, they integrate the 
effects of changes in their environment over very small 
temporal scales. Therefore, nutrient and primary 
production responses in the water column (e.^., by 
phytoplankton) and on the surface sediments (e.^., by 
microphytobenthos) often occur within the time 
period of a few hours to several days. This relatively 
short response time creates a challenge to collect 
measurements in appropriate time scales. A water 
column monitoring approach sensitive enough to 
record bi-weekly or even weekly changes in water 
column productivity could result in a major imposition 
on project budgets and collection logistics. After 
consideration of available resources and funding for 
monitoring activities, sampling of water column 
productivity during the demonstration project was 
scheduled on a monthly basis. This schedule, although 
possibly limiting the ability to assess the immediate 
effects of project diversions on phytoplankton and 
microphytobenthos, provided an opportunity to assess 
changes in broader column productivity characteristics. 



Measurements 

Hydrography 

Physical hydrographic measvirements were made at the 
surface at each sampling site. Parameters recorded 
were sampling location, date, time, latitude, longitude, 
sample depth(s), temperature, salinity, dissolved 
oxygen, per cent oxygen saturation, pH, Secchi depth, 
water depth and weather conditions. A multi- 
parameter YSI model 610 profiler instrument was used 
for /« situ measurements of salinity and depth 
parameters. The units of measure (and their nominal 
accuracy) were: salinity, after conversion from in situ 
conductivity and temperature (0.2 practical salinity 
units (psu)) and depth (1 cm). Salinity was also 
measured by field refractometer in many samples and 
reported as parts per thousand (ppt) for comparison 
purposes. Differences as large as 4 ppt are often 
observed due to the high variability of salinity in 
estuarine waters, the difficulty of maintaining 
calibration in the electronic instrument and keeping the 
field refractometer clean and dry. Dissolved oxygen. 



pH and Secchi depth were not analyzed, but the data 
were logged for the sake of completeness. 

Nutrients 

Ambient Nutrient Concentrations - The 

concentrations of nitrate, nitrite, ammonium, 
orthophosphate and silicate were determined in all 
water samples according to published methods of 
Environmental Protection Agency (1983) and 
Whidedge et al. (1981) using automated continuous 
flow analyzers. All nutrient samples were analyzed 
with a Technician AutoAnalyzer II. The water samples 
were collected in pre-numbered polyethylene bottles 
and immediately placed in the dark and on ice. 
Chemical analysis of the samples occurred within 
24 hours of collection and was often completed within 
5 to 6 hours. Calibration of the automated nutrient 
channels were performed with each set of samples. A 
series of five concentrations for each analyte was 
analyzed prior to analysis of field samples in order to 
ascertain proper operation. A detailed protocol of 
standards and their preparation are described by 
Whidedge et al. (1981) and have been used for 
estuarine/marine samples from 1975 through the 
present. All standards were prepared in the laboratory' 
using either ultra-pure grade deionized distilled water 
or, as a standard addition, low nutrient sea water. 

Nutrient Amendment Bioassay Studies — Bioassay 
techniques were employed in the field to evaluate the 
relative influence of nitrogen, phosphorus, or trace 
metal additions to changes in phytoplankton biomass 
[i.e.. Chlorophyll A). These botde assays were 
enrichment modifications of the productivity estimates 
and are useful to determine possible nutrient 
limitations. The bioassay amendment studies were 
accomplished in screw cap test tubes that contained 
50 milliliters (ml) of sample. Initial samples were 
analyzed for extracted chlorophyll content. Four 
replicates of each sample were amended with 10 micro- 
moles per liter (jimole/l) of ammonium, 10 (omole/l of 
phosphate, 10 jimole/l of ammonium plus 10 ^mole/1 
of phosphate or 100 micro-liters (jj) of "f/2" trace 
metal stock solution. Four replicates of a control 
sample with no additions were also utilized. After the 
additions, the caps were tightened and in vivo 



Chapter Four ♦ 4-3 



fluorescence readings were taken on samples. The 
amended samples were placed in diffuse lighted 
incubators at 25 °C and additional fluorescence 
measurements were taken daily for 3 to 4 days. The 
mean of the four in vivo fluorescence samples was used 
to represent the effect of the amendment additions. 
No readings were discarded. VClien the incubations 
were terminated, the samples were analyzed for 
extracted {in vitro) chlorophyll content. 

Phytoplankton Pigments 

Changes in phytoplankton biomass were determined 
using Chlorophyll A as the index of biomass. The 
chlorophyll and pigment samples were analyzed 
with a model 10-005RU Turner Designs fluorometer 
which was specifically designed for pigment analyses 
using the methodology of Hohn-Hansen et al. (1965). 
Calibration of the in vitro chlorophyll analysis was 
accomplished with pure chlorophyll obtained 
commercially and standardized with a 
spectrophotometer. Phaeopigment concentrations 
were also determined in the same samples after 
addition of a small amount of hydrochloric acid. 

Primary Production 

Rates of phytoplankton primary production were 
monitored using replicate '^C incubations and natural 
sunlight using the method of O'Reilly and Thomas 
(1983). This method has been used for all 
measurements in South Texas bays over the previous 
8 years. The procedure consists of collection of 
duplicate water samples that were inoculated with 
'■"C isotope and incubated in a water bath for 2-3 hours 
in fuU sunlight. Dark botde uptake was measured for 
corrections and '''C inoculation volumes were checked 
with replicate initial blanks. Primary production 
'■"C measurements were analyzed with a Beckman 
model LS5801 liquid scintillation counter that 
employed self-calibration with known sources and 
calculates counting efficiency. Initial carbonate 
alkalimty was analyzed by standardized methodology of 
25 ml of 0.01 M hydrochloric acid additions to 100 ml 
of sample. The extremely high alkalinity of Rincon 
Bayou often required additional aUquots of acid 
addition until a proper pH of <3.9 was obtamed. 



Sedimented Plankton (Microphytobenthos) 

Chlorophyll — The chlorophyll content of sediment 
was determined at each site by sub-sampling a 5-cm 
core collected by hand. A 1-cc syringe was also used to 
collect the sample from the upper 0.5 cm of the 
sediment surface. Extraction and analysis of the 
chlorophyll/phaeopigment content were conducted 
accordifig to the same procedures as the water samples. 

Primary' Productivity — The primary production of 
microphytobenthos was determined on 1-cc mud 
samples from the top of 5-cm cores collected by hand. 
The sediment was suspended in 25 ml of filtered water 
collected at the site. Replicate '''C incubations were 
incubated in natural sunlight for 3 to 4 hours. 



RESULTS 
Hydrography 

Temperature 

Temperature is generally not a strong controUing factor 
on water column primary production, but seasonal 
temperature changes may have a secondary effect on 
production processes. Temperature data were most 
useful in characterizing rapid changes in environmental 
conditions, such as sudden changes of weather during 
winter cooling events [e.g., fironts). Long extended 
periods of high temperature were used to identif)' 
periods of drought and other times of stress on the 
plant and animal populations in the upper delta. 

Salinity 

Salinity is a conservative variable because its 
concentration is altered only by physical processes. As 
precipitation or evaporation occurs, salinity can be 
used to produce an accurate estimate of the quantity of 
water added or subtracted from an estuary. In 
addition, salinity values also give a good indication as 
to the spatial extent of freshwater inflow events. 

In general, the upper Nueces Delta experienced a wide 
variance of salinity concentrations during the 



4-4 V Water Column Productivity 



demonstxation period, ranging from over 120 ppt 
(Station 65) to less than 1 ppt {e.g.. Station 66) 
(Figure 4-2). The incomplete salinity graphs with 
missing data mosdy resulted from dry periods when 
there was no standing water at the stations. The 
Nueces River site (Station 68) had the lowest salinity at 
all times except on September 16, 1999. This sampling 
date was at the conclusion of Event 36 (Chapter 3), 
when a large volume of water previously diverted into 
the Rincon Bayou during the event had flowed back 
into the Nueces River (Figure 3-9) likely transporting 
acquired salt from the upper delta. 

Measurements of freshwater flow into and out of 
Rincon Bayou (Chapter 3), and direct precipitation 
were compared with water column saUruty data 
(Table 4-1 and Figure 4-3). Because salinity at the 
water column stations was measured monthly, daily 
rain and inflow data were summed by water column 
sampling dates. The variations in average salinity at 
each site over time clearly showed the liighly variable 
amounts of precipitation and evaporation over the five 
year sampling period (Figure 4-3). During the months 
following high freshwater inflow periods in summer 
1997 (Events 16 and 17), fall 1998 (Events 23 dirough 



27) and fall 1999 (Events 36 and 37), the upper Rincon 
Bayou site (Stations 65 and 66) had lower salinity 
values than those in the central Rincon Bayou site 
(Stations 61 and 62), and often times lower than the 
Reference sites (63 and 64). This condition {i.e., salinit)' 
concentrations lowest in the upper delta) represents a 
"normal" estuary salinity gradient typically found in 
unperturbed systems. During dry periods and drought 
{e.g., the last part of 1995 through the first part of 1996, 
as well as summer 1998), a "reverse estuary" condition 
was observed {i.e., the highest salinity concentrations 
were found in the upper delta), where the upper 
Rincon Bayou site was predominandy saltier than 
Reference or central Rincon Bayou sites (Figure 4-3). 
These observations conformed closely to the project 
design in that medium to high river flow events 
circulated tiirough the historical Rincon Bayou 
channel. 



Nutrients 

Inorganic nutrients are utilized by plants to produce 
organic matter through the process of photosynthesis. 
The concentrations of nutrients available and amount 





I I I 
1996 



I I I I I I I I I I I I I I I I I I I I I I I I I I I 
1998 1999 



Figure 4-2: Salinity at all water column stations (except Station 62) for each sampling date. Data from Station 
62 were not plotted because the Tidal Flat site was frequently dry resulting in significant data gaps. 



Chapter Four ♦ 4-5 



Table 4-1: Hydrographic events (from Table 3-3) occurring prior to each water column sampling period. Daily 
values for net flow and total precipitation were summed between sampling intervals. In some cases, sampling occurred 
during an event. The Nueces Overflow Channel was completed on October 26, 1995, so flow prior to that date was zero. 
Precipitation data for sampling dates prior to May 16, 1996 were that recorded at the Corpus Christi International Airport. 
Hyphens (-) indicate missing or incomplete data. 



Water 


Hydrographic 


Total Net Flow 


Total 


Water 


Hydrographic 


Total Net Flow 


Total 


Column 


Events 


Through Rincon 


Precipitation 


Column 


Events 


Through Rincon 


Precipitation 


Sampling 


(Table 3-3) 


Bayou 


(inches) 


Sampling 


(Table 3-3) 


Bayou 


(inches) 


Date 




(acre-ft) 




Date 




(acre-ft) 




28-Oct-94 







7.37 


18-Jun-97 




-9 


1.32 


17-NOV-94 







0.20 


14-JUI-97 


16, 17 


1386 


1.95 


07-Dec-94 







2.17 


07-Aug-97 




163 


0.08 


11-Jan-95 


1 





6.35 


17-Sep-97 




13 


0.19 


13-Feb-95 







0.44 


28-Oct-97 


18 


265 


11.36 


15-Mar-95 







6.33 


04-Dec-97 




-6 


2.66 


17-Apr-95 







0.99 


17-Dec-97 







0.08 


16-May-95 







0.48 


27-Jan-98 




3 


0.56 


17-Jun-95 


2 





4.10 


24-Feb-98 




35 


3.01 


20-Jul-95 


3 





1.14 


24-Mar-98 




-3 


2.01 


17-Aug-95 


4 





4.46 


08-Apr-98 


19 


19 


0.00 


27-Sep-95 


5 





4.81 


02-May-98 




21 


0.15 


25-Oct-95 







0.36 


05-Jun-98 




4 


0.00 


28-NOV-95 


6 


- 


12.83 


08-Jul-98 


20 


-9 


0.82 


14-Dec-95 


7 


- 


0.04 


19-Aug-95 


21 


-2 


4.67 


17-Jan-96 


8,9,10 


- 


0.51 


29-Sep-98 


22, 23, 24 


649 


4.39 


17-Feb-96 




- 


0.04 


24-Oct-98 


25 


4246 


9.15 


20-Mar-96 




- 


0.00 


18-NOV-98 


26,27 


130 


3.95 


10-Apr-96 




- 


1.11 


18-Dec-98 




-14 


0.60 


15-May-96 




- 


0.47 


12-Jan-99 




-3 


0.50 


17-Jun-96 




21 


0.66 


24-Feb-99 




-6 


0.51 


21-Jul-96 







1.58 


18-Mar-99 


28 


13 


0.06 


27-Aug-96 


11 


18 


4.94 


15-Apr-99 


29 


130 


2.29 


30-Sep-96 




21 


4.54 


24-May-99 


30,31 


-135 


3.81 


30-Oct-96 


12, 13 


247 


0.80 


09-Jun-99 


32 


2 


1.08 


18-NOV-96 


14 


47 


0.18 


21-Jul-99 


33,34 


15 


6.89 


10-Dec-96 




14 


1.16 


19-Aug-99 


35 


-16 


0.26 


30-Jan-97 




2 


2.32 


16-Sep-99 


36 


821 


7.25 


06-Mar-97 




20 


1.41 


28-Oct-99 


37 


-54 


3.60 


24-Mar-97 




19 


5.05 


17-NOV-99 




-5 


0.00 


21-Apr-97 


15 


-15 


4.95 


08-Dec-99 




18 


0.28 


29-May-97 




33 


5.99 











4-6 V Water Column Productivity 



50 
45 
40 
35 



E 30 
o 



15 
10 

5 -] 




e 

o 



5000 



4000 



3000 - 



2000 



1000 - 




150 



120 



Q. 

>. 

60 ™ 



1995 



1996 



1997 



1998 



1999 



H Total rainfall 

■ Total flow into Rincon Bayou 

■ Average salinity: Reference Site (Stations 63 and 64) 

- Average salinity: central Rincon Bayou Site (Stations 60 and 61] 

- Average salinity: upper Rincon Bayou Site (Stations 65 and 66) 



Figure 4-3: Average salinity at each sampling site for each sampling date. Cumulative daily rainfall and inflovt/ into 
Rincon Bayou also plotted for each monthly period between sampling dates. 



of sunlight often determine the amount of biological 
productivity in an estuary. Organic matter from die 
plants may go through several padiways depending on 
whether it was eaten, decomposed or buried in the 
sediments. Organic matter that is consumed may be 
excreted back into the environment or be incorporated 
into the animal tissue. Microbial populations can also 
absorb or breakdown organic matter and return it to 
the environment. Both organic matter pathways, 
through higher animals or microbes, produce 
regenerated nutrients which enhance nutrient 
concentrations in the water and again become available 
for uptake by plants. The relative amounts of the 
original nutrients and the regenerated nutrients can 
often provide rate process estimates for turnover of 
organic matter in estuaries. 

The relative importance of different nutrients varies in 
freshwater and marine environments. As a result, the 



importance of phosphorus nutrients are often clearly 
obser\^ed in freshwater segments, while nitrogen 
nutrients are most important in saline segments. These 
differences are caused by many physical and biological 
processes that vary over short time and spatial scales. 

Nitrate (NO,) 

Nitrate is the most common form of nitrogen nutrient 
in oxygenated environments. The concentrations of 
nitrate in many rivers has increased over the past five 
decades as a result of increased usage of agricultural 
fertilizer and wastewater effluents. 

The nitrate content of the Nueces River and the 
Nueces Delta stations (Figure 4-4) were relatively low 
(< 2 ^mole/l). Less dian 1% of the data were > 
2 |jmole/l and usually occurred during flooding events. 
The largest concentrations of nitrate (> 20 ^xmole/I) 



Chapter Four *♦* 4-7 



were only found at the Reference site (Stations 63 and 
64) and only during 1999. The relatively low nitrate 
concentrations obsen^ed throughout the data record 
would provide a low level maintenance for 
phjlioplankton growth but would certainly not provide 
nutrients needed to fiiel bloom conditions. 

Ammoniutn (NH4) 

Ammonium is the form of nitrogen most commonly 
released by rec)'cling of organic matter or excretion by 
higher organisms. Accordingly, ammonium is often 
called "regenerated" nitrogen. Shallow environments 
such as the Nueces Delta and Nueces Bay tend to 
increase the relative amounts of regenerated nitrogen 
compared to nitrate (which is often called "new" 
nitrogen because it has been newly added to the 
ecosystem by advection or inflow). 

The concentrations of ammonium observed during the 
demonstration period were usually larger than nitrate 
concentrations (Figure 4-5). The Nueces River station 
(Station 68) often had the smallest ammonium levels, 
while the upper Rincon Bayou stations (65 and 66) 
were sometimes largest. The central Rincon Bayou 
stations (60 and 61) occasionally contained the largest 



ammonium concentrations. There was no obvious 
direct relationship between salinity and ammonium at 
any of the station sites. 

Nitrite (NOj) 

Nitrite is the form of nitrogen that is intermediate 
between nitrate and ammonium for its valence state 
and is not typically observed in large concentrations in 
most estuarine or marine environments. 

During the demonstration period, the small concen- 
trations of nitrite resulted from either nitrification or 
denitrification processes (Figure 4-6). Estuarjr 
ecosystems tend to have both nitrification and 
denitrificaion processes occurring at the same time by 
specific microbes for each. Large concentrations of 
nitrite {e.g., >1 jimole/1) indicate that special conditions 
existed for a short time. 

Dissolved Inorganic Nitrogen (DIN) 

DIN is the sum of the nitrate, nitrite and ammonium 
forms of nitrogen. This amount represents the total 
inorganic nitrogen nutrients available for uptake by 
plants. Because all three forms of nitrogen are readily 





1995 



1996 



1997 



1998 



1999 



Figure 4-4: Nitrate concentrations at all water column stations (except Station 62) for each sampling date. 

4-8 ♦♦♦ Water Column Product! viiy 





I I I I I I 

1999 



Figure 4-5: Ammonium concentrations at all water column stations (except Station 62) for each sampling date. 




T-r 

1998 



Figure 4-6: Nitrite concentrations at all water column stations (except Station 62) for each sampling date. 



Chapter Four ♦ 4-9 



utilized by photosynthetic processes, DIN is often 
used to estdmate nitrogen nutrient availability. 

The concentration of DIN at most stations was 
< 10 ^imole/l, and, on only six sampling dates, it was 
larger than 20 |imole/l, of which five were at the 
Reference site (Figure 4-7). In general, the Reference 
site (Stations 63 and 64) displayed most of the largest 
concentrations of DIN, which likely was a result of 
unusually large inputs from local runoff The Nueces 
River (Station 68) generally had the lowest DIN 
concentrations. 

The largest contributor to the DIN pool was 
ammonium, as indicated by its percentage of DIN 
(Figure 4-8). In only a relatively few samples (about 
6%) was ammonium less than 40% of the DIN. 
During 1999, a relatively large number of samples 
contained 100% ammonium. 



5 |imole/l or lower, although several samples had 
values near 30 |jmole/l (Figure 4-9). 

Also, these concentrations represented high 
concentrations of phosphorus relative to nitrogen. 
For example, most values for the nitrogen to 
phosphorous (N:P) ratio calculated from phosphate 
and DIN concentrations were below 3.0 (Figure 4-10). 
A typical value of the N:P ratio for organic matter is 
about 15. During the demonstration period, only 
5 samples had values larger than 15.0, indicating a 
possible shortage of phosphorus in the system. In 
contrast, there were a large number of samples with 
values < 5.0 for N:P, indicating that phosphorus was 
ver^' abundant and nitrogen may be limiting 
autotrophic processes. 



Phytoplankton Pigments 



Phosphate (PO4) 

Phosphate (orthophosphate) is the primary 
phosphorus nutrient, and it's dynamics vary widely in 
fresh and salt waters. During the demonstration 
period, phosphate concentrations were primarily about 



Phytoplankton pigments are useful indicators of plant 
biomass. Chlorophyll A has been the primary pigment 
traditionally used, but better analytical methods since 
1985 have allowed a large number of other pigments to 
be identified and measured accurately. Chlorophyll A 




-r-rvr I I I I I I I I I T I I I 

1995 1996 



1997 



1998 



1999 



Figure 4-7: Dissolved inorganic nitrogen concentrations at all water column stations (except Station 62) for each 
sampling date. 



4-10 ♦ Water Column Productivity 



100 




I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I 
1995 1996 1997 1998 




1999 



Figure 4-8: Percent of dissolved inorganic nitrogen contributed by ammonium at all water column stations 
(except Station 62) for each sampling date. 





I I I I I I I I I I I I I I I I I I I I I 
1 998 1 999 



Figure 4-9: Phosphate concentrations at all water column stations (except Station 62) for each sampling date. 



Chapter Four ♦ 4-11 



c 
o 



ro 



3 



(/) 
O 

Q. 
O 

C 
Q) 
O) 
O 





1995 



Figure 4-10: Nitrogen to phosphorous (N:P) ratios at all water column stations (except Station 62) for each 
sampling date. 



remains the typical pigment measured by fluorometric 
techniques but is often accompanied by a 
phaeopigment estimate after the addition of acid. 

Chlorophyll 

Chlorophyll concentrations (which are an estimate of 
plant biomass in the water) were quite large during the 
demonstration period, with a large number of values 
> 100 |j,g/l (Figure 4-11). The smallest concentrations 
were mosdy measured in the Nueces River (Station 68), 
but aU other sites had some values > 1 50 (ig/1, which 
could be described as bloom conditions. These 
biomass accumulations of plankton partially drive the 
large rates of primary production in the Nueces Delta. 



Water Column Production 

Water column primary production is a combination of 
phjrtoplankton biomass and the growth rate of 
individual cells. Therefore, combinations of high 
biomass or high growth rates can produce large rates 
of primary production. The rate of primary 
production is a function of nutrient availability and 
incident radiation to the phytoplankton cells. 



The '''C experiment conducted as part of this 
demonstration project indicated that the average rate 
of primary production is about 3 gC/m'/day 
(Figure 4-12), but higher values were typically as large 
as 10 gC/m"/day. One extremely high rate 
(38 gC/m"/day) was obtained at Station 63 before the 
opening of the Nueces Overflow Channel as a result of 
a bloom of filamentous blue-green algae. Frequendy 
the Nueces River station (68) had the lowest rates, but 
all Rincon Bayou stations showed large responses 
during inflow events, particularly during the summers 
of 1997 (Events 16 and 17) and 1999 (Events 29 and 
33). The observed rates of primary production in the 
upper Nueces Delta were larger than values for Nueces 
Bay (StockweU 1989), and the highest rates were equal 
to the largest observed in up-welling areas in the ocean. 

Assimilation Index 

The assimilation index is the rate of primary 
production normali2ed to the chlorophyll biomass. 
This index is useful to determine if the specific growth 
rates of phytoplankton cells are large enough to create 
bloom conditions. Typical assimilation index values of 
50 to 100 mgC/m^/day/fjgChl are observed for many 
phytoplankton spring blooms. 



4-12 



Water Column Productivity 



1000 




1995 



1996 



1997 



1998 



1999 



Figure 4-1 1 : Chlorophyll concentrations at all water column stations (except Station 62) for each sampling date. 




Figure 4-12: Primary production at all water column stations (except Station 62) for each sampling date. 



Chapter Four ♦ 4-13 



The assimilation index for most stations had values of 
20 to 100 mgC/mVday/^gChl (Figure 4-13). Many of 
the highest values obsen^ed before appreciable inflow 
passed through the Nueces Overflow Channel (i.e., 
prior to October 1996) were measured at the Nueces 
River station (68). Later during the demonstration 
period, the upper Rincon Bayou stations (65 and 66) 
were much higher compared to earlier years, and most 
station samples gave higher results during the final 
year. This apparent increase at most of the sampling 
stations during the final year might be due to increased 
freshwater diversions. 



Nutrient Amendment Bioassays 

The nutrient amendment studies were undertaken to 
determine whether phytoplankton growth widiin the 
Nueces Delta could be stimulated by the addition of 
nitrogen, phosphorus or trace metal nutrients. The 
biomass of the phytoplankton as measured by 
chlorophyll was used to indicate biological response. 
The amount of change may be influenced by the 
incubation conditions, but the relative responses can 
provide valuable clues to the degree of limitation of an 
addition or set of additions. Two nutrient amendment 



experiments were performed in March and April 1997 
during what was a relatively dry period in the delta. A 
final amendment experiment was undertaken in August 
1997, after a large amount of summer-time freshwater 
inflow (Events 16 and 17). 

Amendment Series 1: March 7, 1997 

Station 68 — The two phosphate amendment additions 
responded equivalent to the control for the four day 
period (Figure 4- 14a). All other additions showed 
increased chlorophyll concentrations compared to the 
controls. Trace metals, nitrate, nitrate plus 
phosphorus and silicon, and nitrate plus phosphorus 
had approximately twice the response compared to 
ammonium and ammonium plus phosphorus. 

Station 63 — AH amended samples showed a 
chlorophyll response for days 1 and 2 that was 
equivalent to the control with no additions 
(Figure 4- 14b). On day 3, both concentrations of 
phosphate had no effect compared to the control, but 
all other additions enhanced chlorophyll production. 

Station 66 — The two phosphate amendment additions 
showed responses equivalent to the control for the 



O 

CD 
T3 



o 

E, 

X 

0) 
■D 

C 

c 
o 



<fl 

CO 

< 



250 



200 



150 




100 



r T I 1 I I f r I I I 

1995 



1 1 I I T 

1996 



I I I I I I I I I I I I I I I I I I I I I I I I 
1997 1998 1999 




Figure 4-13: Assimilation index at all water column stations (except Station 62) for each sampling date. 
4-14 •♦* Water Column Productivit)! 



(a) Station 68 

5 



% 4 
>^ 

X 

% 3 

c 

0) 

o 

^ 2 

O 
(/) 
0) 



o 

3 



1 - 



DayO 
Day 1 
Day 2 
Day 3 
Day 4 




Control 



NH4 



N03 



NH4+P 



N03+P NH4+P+SI N03+P+SI 



Tm 



O 
>^ 

X 

0) 
•D 

c 

<u 
o 

c 

0) 

o 

CO 

o 

3 



(b) Station 63 

9 
^ 8 
7 
6 
5 
4 
3 
2 
1 




^^ DayO 




1 1 Day 1 
■■■ Day 2 
1 1 Day 3 


1 B 1 




Fl 


III 


1 






ll 




ll 


1, 

ll 


1 

ll 


1 


i " 










ll 


ll 


ll 


■■ i 


ll 


ll 





Control NH4 



N03 



P2 NH4+P N03+P NH4+P+SI N03+P+Si Tm 



(c) Station 66 



o 
>^ 

X 

0) 

(D 
O 

c 

0) 

o 
(/) 

0) 

L_ 
o 

3 



<^4 -| 

00 


^^ DayO 
1 1 Day 1 
^■i Day 2 
r 1 Day 3 












■1R 


1t"^~! 


1 ■- - 1 


1. 


14. 


^ 




1 


pill 


n' 


pii 


11 — lijj 


— 


12 
10 - 

8 - 

6 

4 - 

2 - 

n - 




1 



Control 



NH4 



N03 



P2 



NH4+P 



N03+P NH4+P+SI N03+P+SI 



Tm 



Figure 4-14: Results from nutrient amendment bioassays for selected stations: March 7, 1997. Chlorophyll 
response indicated by fluorescence. Each daily value represents the mean of four replicate samples. 

Chapter Four *■ 



4-15 



three day period (Figure 4- 14c). All other additions 
increased chlorophyll concentrations compared to the 
controls. 

Amendment Series 2: April 22, 1997 



Station 66 — All additions showed an increase in 
chlorophyll compared to the control except phosphate 
additions (Figure 4-15c). VChile all samples showed a 
decreasing trend, the chlorophyO enhancements were 
approximately a 30% increase over the control. 



Station 68 — All additions showed an increase in 
chlorophyll compared to the control except phosphate 
additions (Figure 4-15a). The chlorophyll 
enhancements were approximately doubled compared 
to the control. 



Station 60 — AH additions showed an increase in 
chlorophyll compared to the control except phosphate 
additions (Figure 4-15d). The chlorophyll 
enhancements were approximately double when 
compared to the control. 



Station 63 — All additions showed an increase in 
chlorophyll compared to the control except phosphate 
additions (Figure 4- 15b). The chlorophyO 
enhancements were increased by approximately 50% 
over the control. 



Amendment Series 3: August 7, 1997 

Station 68 — Additions of ammonium, nitrate, silicate 
and ammomum plus P showed mcreased chlorophyll 
responses compared to the control (Figure 4-16a). 



(a) Station 68 

6 



« 5 



X 4 -F 



C 

s 



^ 1 



DayO 
Day 1 
Day 2 
Day 3 



(b) Station 63 

14 T 



•ST 12 



i 10 



o 

lI 2 



DayO 
Dayl 
Day 2 
Day 3 



(c) Station 66 

11 



(d) Station 60 



10 - 



7 



3 
2 - 

1 




DayO 
Day 1 
Day 2 
Day 3 




P NH4+P 



•« 6 
o 

i 5 



£ 2+- 

o 

c 1 4 



DayO 
Day 1 
Day 2 
Day 3 



Figure 4-15: Results from nutrient amendment bioassays for selected stations: April 22, 1997. Chlorophyll 
response indicated by fluorescence. Each daily value represents the mean of four replicate samples. 



4-16 



W^ater Column Productivity 




(b]3tMton» 

40 



30 



I 10 



OayO 
Day4 



OMm NH4 N09 



l'*H4»P 



CiaM NhH 



NC'2 



f*4*tP Bt4 



Tm 



[cf Station M 



» 

K 

20.. 

1» 
10 
& -I 



DayO 
^w^ i 



ll I M 



(d) Stalian 60 
>> 

JO - ■■ 

» 14 • 

1 U 

!K ■ 

e - 
o 
3 * 



DayD 
Day 2 
D8y4 



l 



COIM NhH N03 



WH4+P BM 



Tm 



CoiAOl hH4 



NO! 



NM'tP SN 



Tm 



Figure 4-16: Results from nutrient amendment bioassays for selected stations: August 7, 1997. Chlorophyll 
response indicated by fluorescence. Each daily value represents the mean of four replicate samples. 



Phosphate and trace metals stimulated small 
chlorophyll responses and were probably not 
significant. 

Station 63 — No chlorophyll increases were observed 
with any of the amendment additions (Figure 4- 16b) 

Station 60 — All additions showed an increase in 
chlorophyll compared to the control except phosphate 
additions (Figure 4-1 5d). The chlorophyll 
enhancements were approximately double when 
compared to the control. 

Amendment Series 3: August 7, 1997 

Station 68 - Additions of ammonium, nitrate, silicate 
and ammonium plus P showed increased chlorophyll 
responses compared to the control (Figure 4- 16a). 



Phosphate and trace metals stimulated small 
chlorophyll responses and were probably not 
significant. 

Station 63 — No chlorophyll increases were observed 
with any of the amendment additions (Figure 4-1 6b) 

Station 66 - No chlorophyll increases compared to the 
control were observed with any of the amendment 
additions (Figure 4- 16c). 

Station 60 - Only trace metal additions enhanced 
chlorophyll compared to the control (Figure 4-16d). 
Other additions produced no significant effects in 
chlorophyll concentration. 



Chapter hour ♦♦♦ 4-17 



MiCROPHYTOBENTHIC SEDIMENT BlOMASS 

AND Production 

The shallow water environment of the Nueces Delta 
allows a large fraction of water column phytoplankton 
to settle to the sediment. These benthic cells mav 
become attached to the sediment surface or remain 
unattached to potendally re-suspended with wind 
induced mixing. 

The absence of data during the period with the most 
freshwater inflow (e.^s.. summer 1997 through fall 1999) 
precluded a full assessment of the demonstradon 
project's effects on microphytobenthic communides, 
but some results were observed from the data available. 

Sediment Chlorophyll 

The biomass of microphytobenthic cells is often 
estimated by chloroph\'Ll concentradons, but there is 
no method to determine if the cells are attached and 
growing in place or merely deposited temporarily until 
re-suspension. Therefore, estimates of 
microphytobenthos producdon include both types of 
plankton. 



The sediment chlorophyll declined at all stations during 
the course of the demonstration period (Figure 4-17), 
but no causal mechanisms were apparent. Because two 
stations at the Reference site (63 and 64) also exhibited 
the decline, it is likely that the trend was not related to 
the effects of the demonstration project. 

The ratio of chlorophyll to phaeopigments in the 
sediment varied within a tjipical range during the 
demonstration period (Figure 4-18). The several high 
ratios of chlorophyll compared to phaeopigments at 
Stations 65 and 66 (upper Rincon Bayou site) during 
1996 indicate that chlorophyll production occurred 
more rapidly than did pigment decomposition during 
that period. 

The inventory of chlorophyll in the water column and 
sediment was not closely related to saEnit}', but the 
combined chlorophyll biomass was largest at salinit}- 
concentrations below 60 psu (Figure 4-19). This 
obsen-ation can partially be attributed to reciprocal 
concentrations of high sediment and low w-ater column 
chlorophyll (Figure 4-20). Much of the time, sediment 
chlorophyll was high when water column chlorophyO 
was low, but at other times, low sediment chlorophyll 
occurred when the water column chlorophyll 
concentration was large. 





iqg<i 



lOQA 



inn? 



Figure 4-17: Sediment chlorophyll concentrations at all water column stations (except Stations 62 and 68) for 
each sampling date. 



4-18 



Water Column Productivity 





1995 



1996 



1997 



1998 



1999 



Figure 4-18: Sediment chlorophyll to phaeopigments ratio at all water column stations (except Stations 62 and 68) 
for each sampling date. 



400 - 


• 






E 

"3) inn 


• 







ophyll (m 


• 
•• 


• 

• 




5 
O 

100 - 

4 

- 


* "^ 1 ] 1 ! \ 



20 40 60 80 100 

Salinity (psu) 



120 140 



180 

160 

^ 140 - 

E, 120 

■& 100 
o 

Z 80 



<D 60 



I 40 4 



20 




. 




• 






• t 






•• • 






• 
• 






••f 




'V- 




*fw . 




ftr>:- . • . 



100 



200 



300 



400 



500 



Water column chlorophyll (mg/m ) 



Figure 4-19: Total sediment and water column 
chlorophyll and salinity. 



Figure 4-20: Sediment chlorophyll and water column 
chlorophyll. 



Chapter Four ♦ 4-19 



Microphytobenthic Producrion 

The primar)' producrion of microphytobenthos was 
generally similar to water column rates, although the 
range of water column primary production values was 
more than two times larger (compare Figures 4-21 and 
4-12). There was some indication that sediment 
production was more likely to be high during winter 
months, while water column production tended to be 
highest in the summer, especially at the two Reference 
stations. 

The sediment chlorophyll was not strongly related to 
sediment primary production values (Figure 4-22). 
This indicates that chlorophyll accumulation in the 
sediments did not necessarily dominate the flux rates 
of sediment primary production and was probably 
attributed to a rather short residence time for 
chlorophyll in the sediments before degradation 
occurred. A direct comparison of water column and 
sediment primary production rates indicates that there 
was no correlative relationship (Figure 4-23) and that 
most of the water column rates were 2 to 4 times larger 
than the sediment rates. 



The total combined water column and sediment 
primary production rates did not have a strong 
relationship to salinity (Figure 4-24), but there was 
certainly a general trend that showed the largest 
production rates were at salinity concentrations below 
60 psu. The upper limits of total primary production 
over the entire range of salinity clearly showed an 
inverse relationship with saltmty. 

Benthic Assimilation Index 

The primary production assimilation index for the 
microphytobenthos (Figure 4-25) was not nearly as 
dynamic as the water column index. During only one 
monthly sampling period in the summer of 1995 did 
the sediment assimilation index exceed a value of 
30 gC/m"/day/Chl. This low range probably indicates 
that severe conditions of limited Light, low nutrients 
and possibly high temperatures found in the sediments 
were not conducive to high rates of primary 
production per unit of chlorophyll. 





1995 



1996 



1997 



1998 



1999 



Figure 4-21 : Sediment primary productivity concentrations at all water column stations (except Stations 62 
and 68) for each sampling date. 



4-20 ♦ Water Column Productivity 



E 4 
O 



= 3 
o 

"D 
O 

CO 

E 



2 - 



c 

(D 

E 



_; M ! M 



20 40 60 80 100 120 140 160 180 

Sediment chlorophyll (mg/m^) 




Water column primary productivity (gC/m /day) 



Figure 4-22: Sediment chlorophyll and sediment 
primatv productivity. 



Figure 4-23: Sediment primary productivity and water 
column productivity. 



14 



>■ 12 

CD '^ 
"O 



O 10 

>. 

> 8 
t3 

■D 

2 6 

E 4 4 



TO 

o 2 

1- 





• 








• 
• 








1 

1 


• 




• 


• 




• * 


1 




• 




•• 
• • 


•w 


■ 






•1 








^ 


• ••• 

1 


'••. 


i 1 



20 



40 60 80 100 120 140 

Salinity (psu) 



Figure 4-24: Total primary productivity (sediment and 
water column) and salinity. 



Chapter Four ♦ 4-21 




I I I I I I I I I I I I I I 
1998 1999 



1 1 I I r 




Figure 4-25: Assimilation index at all water column stations (except Stations 62 and 68) for each sampling date. 



DISCUSSION 

The extreme environment of Rincon Bayou is readily 
apparent when the wide ranges of temperature and 
salinit}' are considered. The nutrient content of the 
water column required for primary' production to occur 
was quite variable during the demonstration period. 
The combination of water column inventories of 
nutrients and chlorophyll pigments with instantaneous 
measurements of primary production provided some 
indications of resources available and their rates of 
utilization. However, the relatively small area and 
volume of water contained in the Rincon Bayou 
ecosystem reduced residence times and accelerated 
fluxes through the marsh. The monthly sampling 
schedule was therefore not generally sensitive enough 
to fully analyze the effects of freshwater inflow events 
on phytoplankton production and growth. However, 
even witli the constraints of monthly sampling 
intervals, it was quite apparent that positive water 
column nutrients and primary production effects 
resulted from the demonstration project. 

First of all, nutrient amendment experiments were 
performed in March and April 1997 to ascertain what 
element(s) Hmited phytoplankton production in the 



waters of Rincon Bayou. A subsequent amendment 
experiment was undertaken in August 1997, after a 
large amount of freshwater inflow (Events 16 and 17), 
to determine if nitrogen was still the limiting nutrient. 
Before freshwater inflow, ammonium additions 
increased chlorophyll production compared to control 
samples from several sites. Those experiments also 
indicated that vital trace metals used in "f ' media also 
stimulated chlorophyll production. Additions of 
nitrate also stimulated chlorophyll production to about 
the same extent as ammonium, but very small ambient 
concentrations of nitrate were observed at the 
sampling sites. In August, after a significant inflow of 
riverine fresh water, no nutrient additions increased 
chlorophyll production at any station in the delta, 
indicating that nutrients were no longer limiting 
phytoplankton production. 

These nutrient amendment bioassays were the primary 
indicator of the relatively rapid nutrient utilization and 
phytoplankton response. Initial responses to the single 
and multiple nutrient and/or trace metal additions 
were on the order of a day and continued for 2 to 
3 days. It has been clearly demonstrated from other 
analyses of freshwater inflows that nitrate utilization 
increased primary productivity' in the Nueces River and 



4-22 



Water Column Productivity 



Nueces Bay (AJCTiitledge and Stockwell 1995). 
Therefore, the rapid uptake of nitrate due to freshwater 
inflow very likely occurred in the upper Nueces Delta 
shordy after numerous hydrographic events during the 
demonstration period, even if these responses were not 
readily observed within the (infrequent) sampling 
intervals. 

Second, the total of water column and sediment 
primary production in the delta had an inverse 
relationship with salinity. Although the water column 
contributed the largest fraction of the total ica. 75%), 
the sediment production rates also provided a 
significandy large amount. As salinity values in Rincon 
Bayou declined below 60 psu, the range of primary 
production increased. Therefore, the diversion of an 
estimated 8,810 10' m' (7,142 acre-ft) of water into 
Rincon Bayou during the demonstration period 
lowered salinity concentrations, the osmotic stress on 
individual organisms lowered and almost certainly 
increased primary production in those waters. 

Third, although the increase of N:P was not strongly 
correlated widi inflow events, at least four sampling 
dates had values near or exceeding 1 5 which were likely 
the result of new nitrogen being added to the study 
area by river inflow. In general, the nutrient 
concentrations in the water column, especially 
dissolved inorganic nitrogen, were within the range to 
allow large amounts of primary production. Most 
(50 to 90%) of die DIN was in the form of 
ammonium, which is readily utilized by both water 
column and benthic phytoplankton. The shallow water 
environment enhanced nutrient remineriaUzation, so 
both the ability to utilize and produce nutrients were 
relatively large. The N:P ratio indicated that nitrogen 
was typically the nutrient in lowest concentration. 

Fourth, the species composition of phytoplankton 
apparently remained dominated by primarily small 
diatoms during most of the monitoring period. 
However, several observations of blooms of other 
phytoplankton were noted immediately after 
freshwater inflow events. These blooms were typically 
comprised of single celled blue-green algae (not the 
filamentous cyanobacteria of algal mats) normally 
present in fresh water or very low salinity 



environments. Although t)'picaUy short-lived, 
(persisting no more than a few days), the presence of 
these blooms did not frequentiy occur in the study area 
prior to the demonstration project except under natural 
freshening events that occurred every several years. 
The more frequent presence of these blooms in the 
upper and central Rincon Bayou was an indication that 
the water column ecosystem was showing a more 
typical response to freshwater inflow. 

Finally, the lack of regularly observed chlorophyll 
biomass accumulation was Ukely a result of the rapid 
flux of water through the monitoring area during 
inflow events. However, the occasional obsen^ation of 
high chlorophyll biomass during periods of moderate 
salinity concentrations of 30-60 psu showed that 
phytoplankton could remain in die monitoring area for 
a sufficient time period to accumulate biomass. This 
analysis is supported by the assimilation index, which is 
the rate of carbon growth per unit of chlorophyll. The 
assimilation index for sampling stations in Rincon 
Bayou was often in the range of 50-100 mg C/m2/day 
per unit of chlorophyll, which is a typical value for 
estuarine systems. There were also numerous 
assimilation values > 100 mg C/m2/day per umt of 
chlorophyll at several sampling stations. These values 
indicate that rapid primary production per unit of 
phytoplankton biomass was occurring, especially 
during 1999, which was a consistendy wet year in terms 
of project diversions (Chapter 3). 

In general, the temporal responses of the 
phytoplankton were too rapid to intensely observe 
nutrient accumulation, chlorophyll increases or primary 
production stimulation to specific inflow events. 
Therefore, the nutrient amendment bioassays were the 
primary data to demonstrate immediate phytoplankton 
responses to freshwater inflow events in the Rincon 
Bayou. Future monitoring of nutrient and water 
column primary production should employ automated 
sampling instrumentation that responds to inflow 
events with increased sampling frequency. The 
increased number of samples plus the temporal 
correlation with other recorded variables, such as 
salinity, would gready improve the understanding the 
dynamics and importance of deltaic habitat for 
phytoplankton. 



Chapter Vour ♦ 4-23 



SUMMARY 

Fresh water diverted by the demonstration project 
stimulated primary productivity in the water column 
and on the sediment surface by importing nutrients 
required for plant growth and lowering salinity 
concentrations. During the demonstration period, 
phytoplankton and microphytobenthos rapidly 
responded to the inputs of riverine nutrients with 



increased growth rates and accumulation of biomass. 
The increased primary production rates were especially 
prominent during periods when sahnit}^ was less than 
60 psu. The assimilation index {i.e., the relative amount 
of growth per cell) was also generally higher dunng 
periods of low salinity, indicating that inherent growth 
rates were also increased by project diversions. 



4-24 



Water Column Pnductivity 



PAUL A. MONTAGNA 

Manne Science Institate 
University of Texas, Austin 

RICHARD D. KALKE 

Marine Science Institute 
University of Texas, Austin 

CHRISTINE RITTER 

Texas Water Development Board, Austin 



"The bottom of an estuary regulates or modifies 
most physical, chemical, geological, and 
biological processes throughout the entire 
estuarine ecosystem via what could be called a 
benthic effect." 

*>Y:>3Yetal. (1989) 



CHAPTER FIVE 

Benthic 
Communities 

INTRODUCTION 

The three major habitats in estuarine marshes are the 
vegetated tidal marshes, the water column and the 
sediments. The sediments are both tidal (ranging 
between the tides) and subtidal (below the tidal 
elevation range). Benthos (bottom dwelling organisms) 
live in association with sediments. Benthic 
invertebrates live either in (infauna) or on (epifauna) 
the sediments. Estuarine benthic infauna are 
particularly susceptible to major changes in salinity 
regimes in the environment because of limited mobility 
(Kalke and Montagna 1991; Montagna and Kalke 1992; 
1995; Mannino and Montagna 1997). Freshwater 
species, which tolerate salinity concentrations, ranging 
from to 0.5 parts-per-thousand (ppt), are typically 
found where rivers meet marshes. OUgohahne species 
live in the upper reaches of estuaries where salinity 
ranges from 0.5 to 5 ppt. Brackish or estuarine species 
can accommodate large variations in salinity ranging 
from 5 to 25 ppt. Marine species generally can not 
accommodate salinity values lower than 25 to 30 ppt 
and are limited to the more saline portions of the 
estuary. Salinity is temporally dynamic at any given 
location, changing with floods and droughts. Thus, 
locations of salinity preference zones for various 
organizations change within an estuary throughout the 
year. The interaction between dynamic hydrography 
and salinity preference means the spatial and temporal 
dynamics of benthic infaunal populations are sensitive 
indicators of freshwater inflow effects (Montagna and 
Kalke 1992; 1995). 

Abundance and biomass of infauna may increase if 
nutrient loading from river input is transformed into 
food for benthic animals (Montagna and Yoon 1991). 



Chapter Five ♦ 5-1 



This occurs when nutrients introduced from a river 
stimulate primary production (Deegan et al. 1986; 
Nixon et al. 1986). The primary' production can be 
deposited, but it may also be advected and deposited 
further downstream, potentially increasing benthic 
productivity away from the river inflow source. This 
assumes that fresh water and low salinity do not have a 
negative effect. Salinity stress on physiology (Finney 
1979) and hypoxia (Ritter and Montagna 1999) could 
reduce benthic populations. The net effect of 
freshwater inflow on biological processes {i.e., 
enhanced productivity, recruitment gains and losses ■via 
low-saHnity intolerance) is therefore a function of the 
interaction between physical processes {i.e., 
sedimentation, re-suspension, advection and seawater 
dilution) and chemical processes {i.e., nutrient 
enrichment and cycling). 

If freshwater inflow enhances benthic producti^'ity, 
then increased abundance and biomass should be 
found if inflow were re-introduced into Rincon Bayou 
and the upper Nueces marsh. Benthic infauna are 
useful indicator species in studies of long-term effects, 
because they are relatively immobile and long-Uved 
compared to plankton of similar size. The larger 
macrofauna (organisms greater than 0.5 millimeters 
(mm) in length) and smaller meiofauna (between 0.5 
and 0.063 mm in length) have different ecological roles 
in marine ecosystems (CouU and Bell 1979; Coull and 
Palmer 1984). Therefore, macrofauna and meiofauna 
could respond to freshwater inflow at different spatial 
and temporal scales. Macrofauna, with planktonic 
larval dispersal, indicate effects over larger spatial scales 
and longer temporal scales. Meiofauna, with direct 
benthic development and generation times as short as 
one month, indicate effects over smaller spatial scales 
and shorter temporal scales. Even where meiofauna 
share ecological properties with macrofauna, the 
meiofaunal processes operate on much smaller spatial 
and temporal scales (Bell 1980). 



OBJECTIVES 

1) To assess the effect of the demonstration project 
on benthic infauna biomass, abundance and 
diversit}'; 

2) To assess the response of different trophic levels 
by examining meiofauna and macrofauna; and 

3) To assess the utilization of marsh habitats by 
infaunal species. 



METHODS AND APPROACH 
Study Design 

Increased opportunity for freshwater inflow into the 
study area was accomplished by lowering the Nueces 
River bank leading to Rincon Bayou (Nueces Overflow 
Channel) just east of where U.S. Highway 37 crosses 
the Nueces River (Chapter 1). A Before vs. After/ 
Control vs. Impact (BACI) experimental design (Green 
1 979) was used to determine effects of the 
demonstration project on benthos. Samples were 
taken both before and after the Nueces Overflow 
Charmel was opened. During each sampling period, 
"control" and experimental impact sites were sampled 
(Figure 5-1). An experimental control did not really 
exist, because the system could not be sampled "with" 
and "without" an overflow charmel at the same time. 
In addition, there is large natural variability in 
hydrographic and organismal responses in this 
ecosystem. Therefore, a reference site was chosen that 
reflected changes caused by natural variability but not 
the overflow channel. The site, which was largely 
unaffected by the demonstration project, was 
considered a reference site to the sites affected by the 
project. The BACI design allowed establishment of 
two kinds of reference points to distinguish variability 
caused by the project from natural variability. 

A second component of the experimental design was 
to replicate at the treatment level to avoid "pseudo- 
replication." Pseudo- replication occurs when 
treatments are confounded with replicates (Hurlbert 
1984). For example, if each site were represented by 



5-2 



Benthic Communities 



Northern 
bluff line 



Approximate Scale 
Kilometers 




Lite 



^incon Bayou 



^ central Rincon Bayou Site 

(treatment) 



Figure 5-1: Locations of bentliic sampling stations. The three sites included one reference site (Stations A and B) and 
two treatment sites, upper Rincon Bayou (Stations C and D) and central Rincon Bayou (Stations E and F). 



only one station, then it could not logically be 
concluded that differences between stations were due 

to site differences, as the same magnitude of change 
could have occurred in two different stations within 
the same site. Therefore, two replicate stations were 
assigned to each site (or treatment level) within the 
project. 

The reference site (Stations A and B) was located 
upstream from the overflow channels, reflecting 
natural variability but not effects from project 
diversion (Figure 5-1). Two experimental (treatment) 
sites were also established: 1) the upper Rincon Bayou 
site, which was nearest the overflow channel and was 



most influenced by the Nueces River, and 2) the 
central Rincon Bayou site, which was located farther 
downstream and was less influenced by the river, but 
more by the tide. Two stations were located in the 
upper Rincon Bayou site (Stations C and D), and two 
stations were located in the central Rincon Bayou site 
(Stations E and F). 

The study was a two-way factorial design where the 
main sources of variation were temporal and spatial 
treatments. The response variables measured were 
benthic macrofaunal biomass, abundance and diversity 
and meiofaunal abundance and major taxa diversity. 
Three replicate samples were taken at each station 
during each sampling date. 



Chapter Five ♦ 5-3 



Benthic samples were taken quarterly in January, April, 
July and October of each year. This sampling regimen 
was chosen to compliment the timing of previous and 
current benthic monitoring programs (Kalke and 
Montagna 1991; Montagna and Kalke 1992; 1995; 
Montagna et al. 1993; Martin and Montagna 1995; 
Mannino and Montagna 1997; Ritter and Montagna 
1999). During the initial programs, samples were taken 
monthly (Kalke and Montagna 1991) or bimonthly 
(Montagna and KaUce 1992). It was discovered that 
there were roughly four seasonal events each year, 
including winter and summer lows, and fall and 
summer highs. Based on the cyclical nature of benthic 
recruitment, growth and population losses, it was 
determined that seasonal sampling was sufficient to 
identify annual trends in long-term sampling programs. 
Sampling began (October 28, 1994) one year before the 
Nueces Overflow Channel was excavated (October 29, 
1995), and continued for five additional years (through 
October 28, 1999). Because the first sample of the 
second year was taken early (October 3, 1995), there 
were five pre-treatment samples and sixteen treatment 
samples. 



Measurements 

Hydtography 

The physical hydrographic conditions of the water 
column overlying sediments was measured at each 
station during each sampling period. Measurements 
were collected at the surface and near the bottom and 
recorded on the field log sheet. Conditions recorded 
during sampling included location, date, time, water 
depth and weather conditions. Water quality was 
measured with a multi-parameter instrument (Hydrolab 
Surveyor II). A sonde unit was also lowered to just 
beneath the surface and to the bottom. The 
instruments allowed collection of a variety of water 
quality parameters rapidly. The following parameters 
were read firom the instrument's digital display unit 
(accuracy and units): temperature (± 0.15 degrees 
centigrade (°C)), pH (± 0.1 units), dissolved oxygen 
(+ 0.2 milligrams per liter (mg/1)), specific conductivity 
(± 0.015 to 1.5 (millimhos per centimeter (mmho/cm), 
depending on range), redox potential (± 0.05 millivolts 



(mV)), depth (± 1 meter (m)) and salinity (reported in 
ppt, automatically corrected to 25 °C). 

Benthos 

Sediment was sampled by hand with core tubes to 
measure both meiofauna and macrofauna abundances. 
Macrofauna were sampled with a 6.7-cm diameter tube 
and sectioned at depth intervals of to 3 cm and 3 
to 10 cm; meiofauna were sampled with a 1.8-cm 
diameter tube and sectioned at depth intervals of to 
3 cm only. Samples were presented with 5% buffered 
formalin. In the laboratory, meiofauna were sorted 
on 0.063 mm sieves, macrofauna on 0.5 mm sieves. 
Macrofauna were identified to the lowest taxonomic 
level possible (usually the species level), counted and 
weighed to the nearest 0.01 mg for biomass. Meio- 
fauna were identified to higher taxonomic levels 
(usually phylum, class or order) and counted. 

Biomass of macrofauna was measured by combining 
individuals into higher taxa categories {i.e., Crustacea, 
Mollusca, Polychaeta and others). Samples were dried 
for 24 hours at 55 °C, and weighed. Mollusks were 
placed in 1 N HCl for 1 minute to 8 hours to dissolve 
carbonate shells and washed before drying. 

AH meiofauna and macrofauna data were digitized and 
proof-read. For macrofauna, species diversity was 
calculated by replicate and by pooling aU replicate cores 
for each site. Diversity was calculated using Hill's 
diversity number one (Nl) (Hill 1973). It indicates the 
number of abundant species in a sample and is a 
measure of the effective number of species (Ludwig 
and Reynolds 1988). The effective number of species 
is a measure of the degree to which proportional 
abundances are distributed among species (Hill 1973). 
It is calculated as the exponentiated form of the 
Sharmon diversity index: 



Nl- 



H' 



As diversity decreases, Nl will tend toward 1. The 
Sharmon index is the average uncertainty per species in 
an infinite community made up of species with known 
proportional abundances (Shannon and Weaver 1 949; 
Hutcheson 1970). It is calculated by: 



5-4 



Benthic Communities 



s 


/ \ 




/ \ 


■ 


'=E 


Hi 


In 


^ 




1=1 


. n, 




. «. 





where «, is the number of individuals belonging to the 
rth of J" species in the sample, and « is the total number 
of individuals in the sample. Hill's Nl was used 
because it is units of numbers of species and is 
therefore easier to interpret than most other diversity 
indices. 

All statistical analyses were performed using SAS soft- 
ware (SAS Institute Inc. 1991). tMI data (except when 
calculating diversity) were log-transformed prior to 
analysis. A two-way ANOVA was used to test for 
differences in meiofauna and macrofauna abundance, 
biomass and diversity among treatments and sampling 
dates. Where treatment effects were significant, Tukey 
multiple comparison procedures were used to find 
pairwise, a posteriori differences among sample means 
within a treatment. The Tukey test finds significant 
differences among sample means, while maintaining 
the experimentwise error rate {i.e., the probability that 
one or more erroneous statements wiU be made in an 
experiment) at 0.05 (Kirk 1982). This mediod, 
therefore, ensured that study data were not 
(incorrecdy) analyzed independentiy. 

Community structure of macrofauna species was 
analy^ied by multivariate methods. TTie species data 
were prepared for analysis by making a matrix where 
each row represented an observation of the average 
number of individuals in each station, or station-date 
combination, and each column represented a unique 
species. The data set was multivariate because there 
were more than one species, which were the response 
variables for the analysis. A common problem with 
such matrices is that many of the variables {i.e., 
columns) covary. The covariance can be either positive 
(two or more species responding similarly to a stimuli) 
or negative (two or more species responding in 
opposite fashion to a stimuli). An example of a 
positive covariance is when all species increase in 
response to increased food. An example of a negative 
covariance is where one species competes with or preys 
upon another. Complex interactions among multiple 



response variables requires multivariate analysis to 
illuminate the common patterns in the data set. 

Principal components analysis (PCA) is a multivariate 
method that is also a variable reduction technique. 
PCA is a useful tool because it transforms the species 
data matrix into new variables that can be: 1) mutually 
orthogonal {i.e., the new variables are uncorrelated to 
one another) and 2) extracted in order of decreasing 
variance {i.e., much of the iaformation of the original 
set, like variance, of variables is concentrated in the 
first few principal components (PCS)). The PCS can 
also be used as predictors in regression analysis 
because they are orthogonal and collinearity {i.e., a 
linear relationship between variables) does not exist. 
All multivariate analyses were performed with the 
SAS FACTOR procedure (SAS Institute Inc. 1991), 
using the PC method on the covariance matrix. When 
performing PCA on the covariance matrix, the analysis 
does not treat all the variables as if they have the same 
variance. All count or measurement data was log 
transformed prior to multivariate analysis. 

Results of the PCA are visualized in bivariate plots. 
Generally, only the first two PC factors (PCI and PC2) 
are used in the plots. The results are visualized in two 
ways: as factor patterns and as loading scores. Each 
data set is simply a matrix {i.e., rows of observations 
versus columns of variables). The factor patterns are 
the PC coefficients for each variable or column. These 
vector patterns were used to interpret what PCI and 
PC2 represent by plotting the column heading as the 
symbol for each point. Next, the loading scores for 
each observation were plotted using the site name as 
the symbol for each point. The plot of the loading 
scores allowed visualization of the relationships or 
correlation among the sampling units, stations in the 
present study. 



Chapter Five ♦ 5-5 



RESULTS 

Salinity 

The water column overlying sediments in the study 
area changed on var)'ing temporal scales because of 
natural conditions (e.g., wet and dry periods) as well as 
hydrographic events through the Nueces Overflow 
Channel (Figure 5-2). Salinity ranges during the 
demonstration period were extreme in Rincon Bayou, 
varying from freshwater conditions (< 0.5 ppt) to 
hyper-saline conditions (> 36 ppt). The highest 
recorded salinity was 160 ppt (Station C) during July 
1 996. Salinity trends appeared similar at different 
stations but were not always the same (Figure 5-3). 
For example, during summer 1995, Stations A and B 
had the lowest salinity values while Station E had the 
highest. During summer 1996, Stations C and D had 
the highest concentrations but, during summer 1998, 
Stations A and B had the highest. Floods during the 
summers 1997 (Events 16 and 17) and 1999 (Events 33 
and 34) maintained salinity values much lower than 
during the other summers. 

In the analysis of the hydrographic effects of the 
demonstration project (Chapter 3), a total of 37 events 
were identified during the study period (Table 3-3). 
Measurements of freshwater flow into and out of 
Rincon Bayou, as well as direct precipitation, were 
compared with benthic salinity data. Because salinity at 
benthic stations was measured quarterly, daily rain and 
inflow data were summed by sampling date (Table 5-1). 
Although data from the Rincon gauge was not available 
prior to May 1, 1996, flow into Rincon Bayou prior to 
the opening of the Nueces Overflow Channel was zero 
(Chapter 3). During the period after the overflow 
channel was exca%'ated but before the Rincon gauge 
was in place, there were some small exchange events 
(e.g.. Event 6, which resulted from locally hea\7 
rainfall), but these were not considered to result m 
substantial inflow into Rincon Bayou (Chapter 3). 
Over the entire period, rainfall events occurred 
frequendy, but there were only four sampling dates 
prior to which significandy large inflows of fresh water 
were recorded (Table 5-1), includingjuly 1997 (Events 
16), October of 1997 (Events 17 and 18), October 
1998 (Events 21 dirough 25), and October 1999 




Figure 5-2: View of benthic Station C in upper Rincon 
Bayou under dry (above) and wet (below) conditions. 

The dry conditions were during the summer of 1996. and 
the flooding conditions during the summer of 1997 
(Event 16). 

Photo courtesy of the University of Texas Marine Science 
Institute. 



(Events 35). Only four of the 37 events identified 
during the demonstration period (Events 16, 18, 25 
and 36) were sufficiendy large to result in delta inflow 
without the demonstration project and, of these, only 
one did so appreciably (Ev^ent 25) (Chapter 3). 

Rainfall and freshwater inflow data were used to 
determine the cumulative effects of dilution on the 
average sahmty at each site (Figure 5-4). Hj^ersaUne 
conditions likely resulted from near zero flows and 
high evaporation rates, which typically occurred in 
summer. The differences between the two treatment 
sites indicate that "reverse estuar)'" conditions had 
existed in Rincon Bayou before freshwater diversions 



5-6 



Benthic Communities 





180 




160 




140 




120 


3 


100 


-1— » 

C/5 


80 
60 




40 




20 









1995 



1996 



1997 



2000 



Figure 5-3: Salinity at all benthic stations (A through F) for each sampling date. 



began (/>., higher salinity concentrations in the delta 
than in the bay). Early in the study period, the upper 
Rincon Bayou site often exhibited higher salinity values 
than did the central Rincon Bayou site, but later in the 
demonstration period, this relationship reversed 
(Figure 5-4). Also, a comparison between the 
reference site and the upper Rincon Bayou site indicate 
the influence of freshwater diversions on the later. 
Before diversions began {i.e., October 1996), the 
reference site (which is subject to runoff from rainfall 
northwest of Highway 77) was predominandy fresher 
than either of the two treatment sites in Rincon Bayou. 
Beginning with the inflow occurrence of fall 1996 
(Event 12 through 14), the upper Rincon Bayou 
reference site was predominandy fresher than the 
reference site (Figure 5-4). 



Temperature 

Temperature rose in summer to 33 to 40 °C and 
dropped in winter to about 12 °C following an 
expected seasonal pattern. Generally, Stations E and F 
had the highest temperatures, and Stations A and B 
had the lowest. Although absolute differences in water 
temperature among stations were quite small, the 
differences are likely due to differences in water depth 
because greater volumes of water change temperature 
more slowly. There was considerable variation in 
station differences between years. Overall, 
temperature differences among stations were not 
sufficient to cause differences in benthic responses. 



Chapter Five ♦ 5-7 



50 -, 

45 

40 

35 -] 



E 30 - 

^ 25 4 

C 

OJ 20 



15 

10 

5 

J 



5000 



- 140 




1995 



1996 



1997 



1998 



1999 2000 



^m Total flow into Rincon Bayou 

I Total rainfall 
▼• • Average salinity: Reference Site (Stations A and B) 
-o— Average salinity; upper Rincon Bayou Site (Stations C and D) 
-a — Average salinity: central Rincon Bayou Site (stations E and F) 



Figure 5-4: Marsh-wide average salinity at each sampling site for each sampling date. Cumulative daily rainfall and 
inflow/ into Rincon Bayou also plotted for each quarterly period betw/een sampling dates. 



Dissolved Oxygen 

Dissolved oxygen (DO) concentrations varied widely 
during the study period, from hypoxic (< 2 rng/l) to 
super-saturated (Figure 5-5). Stations A and B were 
most likely to be hypoxic. These stations have sea 
grasses, so decay of organic matter or night time 
respiration likely caused the hypoxia, which was mostly 
likely to occur in summer. The solubility of oxygen in 
sea water decreases with increasing temperature and 
salinity, both of which increased in summer. 



Benthos 

To indicate the importance of freshwater inflow, the 
average salinity at all stations (instead of at each site) 
was evaluated against many organismal response 
variables. 



Macroinfauna 

There was a strong significant interaction (P = 0.0001) 
between stations and dates for both log-transformed 
biomass and log-transformed abundance. Station C, 
the most directiy affected by diversion from the 
Nueces Overflow Channel, had the highest biomass 
and abundance of macrofauna (Table 5-2). However, 
because of the significant interactions, differences 
among stations means could not be determined with a 
simple post hoc comparison, and examination of the 
interaction itself was necessary. 

At various times. Stations E and F had the lowest 
biomass, or Stations A and B had the lowest 
(Figure 5-6a). Station C had the highest abundance of 
all stations during periods of peak biomass blooms. 
Station C was the station most directly influenced by 
inflow from the project. There was a strong seasonal 



5-8 



Benthk Communities 



Table 5-1: Hydrographic events (from Table 3-3) occurring prior to each benthic sampling period. Average 
salinity concentrations reported for all benthic stations (A through F). Daily values for net flow and total precipitation 
were summed between sampling intervals. In some cases, sampling occurred during an event. The Nueces Overflow 
Channel was completed on October 26, 1995, so flow prior to that date was zero. Precipitation data for sampling 
dates prior to May 16, 1996 were that recorded at the Corpus Christi International Airport. Hyphens (-) indicate 
missing or incomplete data. 



Benthic 
Sampling Date 


Hydrographic Events 
(Table 3-3) 


Total Net Flow Through 

Rincon Bayou 

(acre-ft) 


Total Precipitation 
(inches) 


Average Marsh-wide 

Salinity 

(ppt) 


28-Oct-94 







15.61 


10.6 


11-Jan-95 


1 





8.68 


15.5 


12-Apr-95 
12-Jul-95 


2,3 






7.72 
5.72 


18.0 
61.7 


3-Oct-95 


4,5 





9.50 


34.6 


8-Jan-96 


6, 7, 8, 9, 10 


- 


13.58 


27.8 


9-Apr-96 
12-JUI-96 




: 


1.15 
2.78 


73.2 
112.6 


22-Oct-96 


11, 12, 13 


260 


10.05 


38.1 


6-Jan-97 


14 


86 


3.06 


48.0 


23-Apr-97 
2-JUI-97 


15 

16 


26 

782 


12.24 
9.26 


14.0 
2.6 


28-Oct-97 


17, 18 


1,065 


11.54 


3.1 


16-Jan-98 




-6 


3.37 


21.3 


8-Apr-98 
9-Jul-98 


19 
20 


54 

15 


5.04 
1.12 


20.7 
75.0 


28-Oct-98 


21,22,23,24,25 


3,900 


18.53 


0.8 


12-Jan-99 


26,27 


161 


4.58 


19.6 


14-Apr-99 
7-Jul-99 


28,29 
30,31,32,33,34 


136 
-90 


2.86 
10.64 


15.7 
5.2 


28-Oct-99 


35 


709 


12.25 


11.6 



Note: 1 acre-ft = 1.2336 10' m'; 1 inch = 2.54 cm 



Table 5-2: Average benthos characteristics at a 


II stations during the 


demonstration period. 








Macrofauna 




Meiofauna 


Station 


Biomass 


Abundance 


Diversity 


Abundance 




(g/m^) 


(n/m') 


(N1 0.1 8/m') 


(n/IOcm') 


A 


1.82 


24,900 


2.1 


433 


B 


2.31 


21,200 


2.2 


369 


C 


3.23 


43,700 


1.5 


1,240 


D 


2.10 


35,700 


1.5 


1,060 


E 


2.42 


24,500 


1.9 


1,540 


F 


1.83 


20,300 


1.7 


1,730 



Chapter Five ♦ 5-9 




2000 



Figure 5-5: Dissolved oxygen at all benthic stations (A through F) for each sampling date. 



trend, as the highest biomass occurred in spring 
(April), and lowest biomass in summer (July) or fall 
(October). 

There was also a strong significant interaction (P = 
0.0001) between stations and dates for log-transformed 
macro fauna abundance (Figure 5-6b). The nature of 
this interaction was much more complex than for 
biomass. No one station appeared to have the highest 
peaks, but Stations A and B (reference site) had almost 
always the lowest values. Again, low values had a 
tendency to occur in summer and higher abundances in 
fall or winter. This result indicated that Stations A and 
B, most removed from demonstration project effects, 
suffered from the consequences of evaporation and 
concomitant high salinity concentrations. 

Macrofauna diversity was very low, ranging from zero 
to 11 species found at a station (Figure 5-6c). Because 
diversity was so low, all three replicates were pooled 
for species analyses lea\'ing no replication. An 
interaction between stations and dates was e'vident. 
However, in 13 of 21 sampling periods. Stations A or 
B had the highest diversit}'. Diversity was highest 
when abundance was highest. 



The seasonal and inter-annual trends were most 
evident when all samples at all stations were averaged 
together to form a marsh-wide average biomass (Figure 
5-7a), or marsh-wide average abundance (Figure 5-7b). 
The marsh-wide averages were most useful because 
station differences were obscured by interaction effects 
with sampUng dates. VCTien the marsh- wide averages 
were compared to average sahnit}', the effect of inflow 
became apparent. VCTien salinit)^ values were high, 
biomass decreased, often to near zero. During periods 
following salinit\' declines due to inflow, biomass 
increased as ex'idenced in the summer through fall of 
1997 and 1998. In contrast, salinit}' declines due to 
rainfall alone did not have the same effect. During 
April 1997, there was a large local rainfall event but 
Htde inflow (Table 5-1, Figure 5-4). Biomass continued 
to decline during spring and summer 1997. In 
contrast, when salinity declined due to an inflow event 
in fall 1997 (Event 18), biomass increased. There was 
a strong seasonal signal, with highest biomasses in 
January' or April of each year, and lowest biomasses in 
July of each year. Another important trend was less 
variability in biomass fluctuations through time as the 
demonstration project progressed. 

The marsh-wide average abundance trend (Figure 5-7b) 
was very similar to the biomass trend described 



5-10 



Benthic Communities 



I Average macrofauna biomass (each station) 




1995 



1996 



1997 



1998 



1999 



2000 



(b) Average macrofauna abundance (each station) 

225000 

200000 

""e 175000 

g. 150000 

3^ 125000 

■^ 100000 

■g 75000 
> 

T3 50000 

25000 





1995 



1996 



1997 



1998 



1999 



2000 



(c) Average macrofauna diversity (each station) 




1995 



1996 



1997 



1998 



1999 



2000 



Figure 5-6: Average macrofauna biomass (a), abundance (b) and diversity at each station (A through F). 

Diversity values represent the mean of three samples at each station. 

Chapter Five ♦ 5-11 



(a) Average macrofauna biomass (all stations) 
8 
7 



160 




1995 



1996 



1997 



1998 



1999 



2000 



a. 
a. 

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'c 
"to 

C/3 



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90000 
80000 



— • — Abundance 
Salinity 



180 
160 
140 




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"(5 

C/5 



1995 



1996 



1997 



1998 



1999 



2000 



(c) Average macrofauna diversity (all stations) 



E 
CO 3 



— •— N1 Diversity 
Salinity 




140 



1995 



1996 



1997 



1998 



1999 



2000 



Q. 

>> 

'c 
15 

05 



Figure 5-7: Marsh-wide averages of macrofauna biomass, abundance and diversity for all stations. 

Salinity values are averaged from all stations for each sampling period. 



5-12 



Benthic Communities 



previously. Peak abundance occurred after inflow, 
lowest abundance during peak salinity concentrations. 
Blooms could result in very high abundances, greater 
than 40,000 individuals per m". This ecosystem was 
characterized by dominance of only a few species. The 
diversity index N I was used to calculate the average 
number of dominant species among all stations at each 
sampling period, and then was compared to salinity 
(Figure 5-7c). Generally, no more than two dominant 
species were present at each sampling period. 
Diversity, measured as the number of dominant 
species, increased following periods of low salinity and 
decreased when salinity was high. 

Macrofauna characteristics have a strong non-linear 
relationship with saUmty, which appeared to be a bell- 
shaped curve skewed to the left with a long tail to the 
right (Figure 5-8). 

A three parameter, log normal model: 

Y^a * exp( -0.5 * (ln(X/ c)/by) 

was used to characterize the nonlinear relationship 
between biological characteristic (1) and salinity (X). 
The three parameters characterize different attributes 
of the curves, where a is the maximum value, b is the 
skewness (or rate of change) of the response as a 
function of salinity, and c the location of the peak 
response value on the salinity axis. The models fit the 
data reasonably well, indicated by the coefficient of 
variation for each parameter ranging from 7% to 31% 
(Table 5-3). Using these parameters, abundance 
appeared to peak at a high salinity around 32.7 ppt. 



biomass at 18.7 ppt and diversity at 9.1 ppt. Lower 
skewness (b) parameters indicates more narrow ranges 
of responses values with respect to salinity. For the 
three characteristics, the salinity range of response (b) 
increased as the salinity peak value for the response 
increased (c) (Table 5-3). 

The direction of salinity change during a sampling 
penod was important for diversity (Figure 5-8c), but 
not biomass (Figure 5-8a) or abundance (Figure 5-8b). 
The lowest values of all biological responses occurred 
at the highest salinity concentrations, and the highest 
concentrations always occurred during periods of rising 
salinity (note the circle symbols in Figure 5-8). Low 
diversity occurred when salinity values were decreasing 
(obser\^ations on graph with diamond symbols) and 
high diversity occurred when salinity values were rising 
within normal salinity ranges (i.e., < 35 ppt). 

In spite of the low average diversity on any given 
sampling date, a total of 37 species were found over 
the five year period of the study (Table 5-4). The 
polychaete, Streblospio benedicti (Figure 5-9) was an 
overwhelmingly dominant species at all stations and in 
the marsh overall (Table 5-5). In fact, S. benedicti 
represented 84% of all individuals found over the 
entire course of the study. Only four other species 
contributed as much of 2% of the community: the 
polychaete Laeonereis culveri, the snail Assiminea sucdnea, 
and unidentified species of ostracod, and unidentified 
chironomid larvae. 



Table 5-3: Parameters from nonlinear regressions to predict macrofauna characteristics from salinity 
(Figure 5-8). Coefficient of variation for parameters in parentheses. 



Characteristic 



Parameter 



Abundance 
Biomass 
N1 Diversity 



45,774 (25%) 
3.426 (15%) 
2.361 (7%) 



0.6663(31%) 

0.9048 (27%) 

1.699(13%) 



32.7(17%) 
18.7(24%) 
9.08(19%) 



Chapter Five ♦ 5-13 



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100 



120 



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o 



o 
o 

Q. 



n 

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Salinity (ppt) 



Figure 5-8: Relationship between average macrofauna characteristics and salinity. Circles (o) represent rising 
salinity, and diamonds (< ) represent falling salinity. Line is fit with a three parameter log normal regression model. 



5-14 



Benthic Communities 



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Chapter Five ♦ 5-15 



Table 5-5: Species overall dominance. Percent of individuals in each sample. 



Species Code 


Taxa Name 


81 


Streblospio benedicti 


491 


Laeonereis culveri 


381 


Assiminea succinea 


181 


Ostracoda (unidentified) 


487 


Chironomid larvae 


562 


Mediomastus ambiseta 


478 


Paranais grandis 


364 


Berosus sp. 


111 


Capitella capitata 


162 


Mulinia lateralis 


307 


Ceratopogonid larvae 


8 


Oligochaetes (unidentified) 


460 


Hemicyclops sp. 


71 


Polydora ligni 


734 


Damselfly nymphs 


201 


Corophium louisianum 


387 


Corophium sp. 


371 


Tricho corixa sp. 


7 


Nemertinea (unidentified) 


557 


Rictaxis punctostriatus 


595 


Hydrophilidae (unidentified) 


854 


DIptera (unidentified) 


494 


Chironomid pupae 


903 


Pentneura sp. larvae 


585 


Mesocyclops sp. 


488 


Macoma mitchelli 


802 


Latonopsis occidentalis 


428 


Mysidopsis sp. 


493 


Mysidopsis almyra 


345 


Tipulid larvae 


689 


Diosaccidae sp. 132-MG 


232 


Callinectes sapidus 


202 


Gammarvs mucronatus 


312 


Psychodid larvae 


880 


Chaoboms sp. larvae 


906 


Helodidae larvae 


905 


Dragonfly nymphs 



Percent 


Commutative % 


84.4371 


84.4371 


4.9874 


89.4245 


2.9154 


92.3399 


2.4330 


94.7729 


2.3698 


97.1427 


0.7671 


97.9098 


0.4692 


98.3790 


0.2636 


98.6426 


0.2557 


98.8983 


0.2030 


99.1013 


0.1766 


99.2779 


0.1028 


99.3807 


0.0923 


99.4730 


0.0791 


99.5521 


0.0712 


99.6233 


0.0501 


99.6734 


0.0474 


99.7208 


0.0448 


99,7656 


0.0343 


99.7999 


0.0343 


99.8342 


0.0316 


99.8658 


0.0290 


99.8948 


0.0132 


99.9080 


0.0132 


99.9212 


0.0105 


99.9317 


0.0053 


99.9370 


0.0053 


99.9423 


0.0053 


99.9476 


0.0053 


99.9529 


0.0053 


99.9582 


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99.9608 


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99.9634 


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99.9660 


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99.9686 


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99.9712 


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99.9738 


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99.9764 



The communities within the upper and central Rincon 
Bayou treatment sites were dominated by a few 
common species, evidenced by the stations clustering 
together on the first axis of the principal component 
analysis (TCA) (Figure 5-10). The PCA 1 axis, or first 
principal component (PCI), explained 70% of the 
variance in the data set, and PCA 2 axis (PC2) 
explained an additional 19% of the variance. The 



dominant organisms Streblospio benedicti (Sb), Laeonereis 
culveri (Lc), unidentified Ostracoda (Os) and 
Chironomid larvae (Ch) drove the trend for PC 1 
toward high positive values because they were part of 
the average dominant commumty (Figure 5-lOa). 
Thus, all stations had high PCI values (Figure 5-lOb). 



5-16 



Benthic Communities 




Figure 5-9: Steblospio benedicti. This benthic organism 
is approximately 1 cm long. 

Photo courtesy of the University of Texas Marine Science 
Institute. 



Other than the four dominant species, all other species 
were rare. The rare species were responsible for 
regional and station clustering along the PC2 axis. 
Presence oi Assiminea sucdnea (As) at Stations A and B 
and oi Mediomastus amhiseta (Ma) at Stations E and F 
were primarily responsible for separating stations. 
Stations A and B also had a higher incidence of insect 
larvae like Chironomid larvae (Ch) , Berosus sp. (Be), 
Ceratopogonid larvae (Ce) and Damselfly nymphs 
(Da). The three clusters (A and B, C and D, E and F) 
indicated the communities within the treatments were 
slighdy distinct from one another. The stations within 
a treatment site {i.e., upper and central Rincon Bayou) 
were more similar than the treatment sites themselves 
(Figure 5- 10b). The community structure data 
presented in the PCA plots were the only macrofauna 
data that showed a strong treatment-site trend. 

The six most dominant species (Table 5-5) were found 
continuously throughout the study, except when 
salinity concentrations were high (> 35 ppt) 
(Table 5-6). Rare species generally occurred during low 
salinity periods only. The only species to occur 
consistentiy during hyper-saline conditions was the 
insect, Triio corixa (SP 371), but it was also found when 
salinity values were brackish, so it was not considered 
an indicator species of freshwater inflow. Each 



drought period {e.g., the summers of 1995, 1996 and 
1998) appeared to be characterized by different species 
(Table 5-6). 

Meiofauna 

The meiofauna community was composed of 
Nematoda, Copepoda (primarily Harpacticoida) and 
16 other taxa. Nematodes comprised 71% of all 
organisms on average, and copepods comprised 9% 
(Table 5-7). Insect larvae comprised only 0.02% of the 
organisms found. The other metazoan taxa comprised 
only 4% of all other organisms found and included 
permanent meiofauna (Turbellaria, Gastrotricha, 
Tardigrada, Cnidaria, Rotifera and Kinorhyncha) and 
temporary meiofauna (Polychaeta, OUgochaeta, 
Gastropoda, Bivalvia, Ostracoda and Amphipoda). In 
addition, two groups of protozoans were found among 
the meiofauna, Ciliata and Foraminifera, which 
comprised about 1 5% of all meiofauna found. 

The average total number of meiofauna was 908,000 
individuals per m^ (Table 5-2). There was a significant 
interaction between stations and dates (P = 0.0001). In 
contrast to macrofauna, meiofauna abundance 
exhibited differences among treatments (Table 5-2, 
Figure 5-1 la). Stations A and B always had the lowest 
abundances. In fact, the average abundance at Stations 
A and B was almost three times lower than the average 
at Stations C and D, and four times lower than the 
average at Stations E and F. 

The average abundance of meiofauna among all 
stations at each sampling period changed with 
changing salinity conditions (Figure 5-1 lb). 
Abundances were lowest when salinity concentrations 
were highest, and recovered after periods of low 
salinity. In general, the pattern was similar to the 
pattern for macrofauna abundances. The lowest 
abundances were recorded during the dry periods of 
1996. After significant freshwater inflow in 1997 
(Events 16, 17 and 18), which lowered salinity, 
abundances recovered and reached the highest level in 
January 1998. 



Chapter Five 



5-17 



(a) Species loadings 

3 




(b) Station score 



< 

O 
Q. 



% 

E 
F 


A 
B 







Abbreviation Key for species: 

Sb = Streblospio benedicti 

Lc = Laeonereis culveri 

As = Assiminea succinea 

Os = Ostracoda (unidentified) 

Ch = Chironomid larvae 

Ma = Mediomastus ambiseta 

Pg = Paranais grandis 

Be = Berosus sp. 

Cc = Capitella capitate 

W = Mulinia lateralis 

Ce = Ceratopogonid larvae 

Ol = Oligochaetes (unidentified) 

He = Hemicyclops sp. 

PI = Polydora ligni 

Da = Damselfly nymphs 

Co = Corophium 



-1 1 

PCA2 

Figure 5-10: Principal components analysis of 16 most common species, including species loadings (a) and 
station score plots (b). 



5-18 



Benthic Communities 



(a) Average meiofauna abundance (each station) 
10000 




1995 



1996 



1997 



1998 



1999 



2000 



(b) Average meiofauna abundance (all stations) 



5000 



120 



- 100 







Figure 5-11: Average meiofauna abundance foreacli station (a) and marsh-wide (b). Salinity 
values are averaged from all stations for each sampling period. 



ChapterFive ♦ 5-19 



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5-20 



Benthic Communities 



Table 5-7: Composition of the meiofauna community. 



Taxa 



Percent 



Nematoda 
Copepoda 
Other metazoa 
Insects 
Protozoa 



71,10 
9.39 
4.17 
0.02 

15.32 



DISCUSSION 

Macrofauna and Meiofauna 

The upper Nueces Delta is an unusual marsh, with 
extreme environmental variability. The variability is 
evidenced by wide ranges in salinity (from to 
160 ppt), high temperatures (12 to 40 °C) and a 
tendency toward reverse salinity gradients, with higher 
salinity concentrations in the upper marsh area. These 
environmental extremes drive the response patterns of 
benthic resources. Macrofauna diversity in Rincon 
Bayou was generally lower than found in either Nueces 
Bay or Corpus Chris ti Bay. The average number of 
dominant species (diversity index Nl) per station was 
only 1.8 in Rincon Bayou, whUe, in contrast, the 
average Nl value was 3.5 in Nueces Bay stations 



(Mannino and Montagna 1997) and 7.0 in Corpus 
Christi Bay (Ritter and Montagna 1999). For 
comparison, hypoxic stations in Corpus Christi Bay 
averaged an Nl value of 1.5 (Ritter and Montagna 
1999). The low diversity in Rincon Bayou reflects a 
relatively greater degree of stress on benthos caused by 
the higher environmental variability, particularly in 
salimty extremes. 

Macrofaunal abundance and biomass, which are 
indicators of productivity, were in a range typically 
found in Nueces and Corpus Christi bays. This 
observation indicates that environmental variability 
affected benthic community structure but was not 
likely to affect secondary productivit}'. However, 
marsh habitats typically have greater productivity than 
open bay habitats (Day et al. 1989), so it was not 
known if production in the Nueces marsh was optimal 
or suboptimal. 

Overall, there is strong evidence that the demon- 
stration project increased producti^^ty and ameliorated 
stresses on biodiversity. These positive effects were 
caused by increased opportunities for freshwater 
inflow into the marsh and responses of the benthos to 
this inflow. Seasonal increases of biomass occurred in 
spring, when salinity values were lowest and water 



100000 



ri"" 80000 

E 

(]} 
o 

g 60000 

C 

J3 
CD 

ro 

c 



O 
03 



40000 



20000 



- Macrofauna 

- Meiofauna 




I- 4000 E 

o 



5000 



X 

c, 

0) 

o 

c 

03 

■o 

X) 
03 

03 

c 

03 



1000 -§ 



- 3000 



- 2000 



1995 



1996 



1997 



1998 



1999 



2000 



Figure 5-12: Marsh-wide average abundance of meiofauna and macrofauna. 



Chapter Five ♦ 5-21 



levels were highest. In contrast, during summer when 
salinity values were highest and water levels were 
lowest, biomass was always lowest (Figure 5-7). Inflow 
events triggered bursts of productivity as indicated by 
increased abundance and biomass following periods of 
lower salinity concentrations. Biodiversity increases 
just after inflow events and as salinity was rising again 
indicated more species were utilizing the marsh habitat 
as a result of increased inflow events, although 
different species appeared after each inflow event 
(Table 5-6). The responses to inflow were found by 
following changes after inflow events that filled Rincon 
Bayou with fresh water. 

Interestingly, prior to overflow channel construction, 
brackish conditions in April 1995 resulted in decreased 
diversity. In contrast, after the Nueces Overflow 
Channel was excavated, brackish conditions in April 
(1997 to 1999) resulted in increased diversity 
(Table 5-6). The responses of increased abundance to 
inflow by macrofauna and meiofauna were similar 
(Figure 5-12), indicating that both trophic levels of 
benthos were responding to inflow events through the 
overflow channel. 

The mechanisms of infaunal response to inflow were 
likely trophic as well as physiological. The physio- 
logical responses were controlled by increased 
survivability and tolerance to specific salinity ranges 
caused by inflow events. Trophic responses were 
indirect and related to responses by potential food 
items. When primary producers responded to inflow 
by increased biomass, then increased food levels led to 
increased secondary production. Therefore, the 
trophic link between primary producers and secondary 
consumers was demonstrated by correlated, or lagged, 
abundance patterns. 

Standing stocks of macrofauna and chlorophyll were 
somewhat concordant. Peaks of macrofauna biomass 
followed periods of increasing chlorophyll 
(Figure 5- 13a). The only exception was in summer 
1997, when meiofauna abundance was more 
concordant with chlorophyll biomass (Figure 5- 13b). 
Peaks of meiofauna abundance followed periods with 
high concentrations of chlorophyll in the overlying 
water. Meiofauna are known to be grazers and to 



favorably respond to the presence of chlorophyll 
(Montagna 1995; Montagna et al. 1995). In 
San Antonio Bay, meiofauna respond to freshwater 
inflow and increased chlorophyll with increased grazing 
rates (Montagna and Yoon 1991). 



Trophic Links 

There was evidence (Riera et al. 2000) that the 
demonstration project also restored the function of the 
Nueces marsh as a nursery habitat for development of 
juvenile brown shrimp, Penaeus a^ecus. Brown shrimp 
spawn offshore in the Gulf of Mexico. Post-lar%'ae are 
carried by on-shore water movement and enter bays, 
ultimately finding productive shallow estuarine waters 
protected from storms and predators (Day et al. 1989). 
Most of the larval brown shrimp enter marine bays 
from late winter through early spring, spend about 
three to four months in estuarine nursery grounds and 
return to the offshore Gulf of Mexico in early summer 
(Moffett 1970). 

As a sub-component to this benthic analysis, the 
trophic dynamic links and migratory behavior of 
juvenile brown shrimp were investigated from Aransas 
Pass to Corpus Christi Bay to Nueces Bay and to the 
Nueces Delta (Riera et al 2000). Stable isotopes ratios 
of carbon and nitrogen (6"C and 6'^N) of shrimps 
and their potential food sources were measured 
between December 1995 and July 1996. Stable 
isotopes of carbon and nitrogen change as a function 
of the food an organism eats (DeNiro and Epstein 
1978; Fry and Parker 1979). Because food sources 
change in different habitats, stable isotopes can also be 
used to assess migration of shrimp (Fry 1981). 

During the study, shrimp lengths increased from 10 to 
1 1 mm when the animals entered Corpus Chnsti Bay 
as larvae, to 80 to 90 mm when they returned to the 
Gulf of Mexico as subadults. Brown shrimp exhibited 
spatial and temporal 6"C variation (from -25.2 to 
12.5%o), indicating a high diversity of food sources 
throughout their migration. From examination of the 
6''C values, it appears the main food sources used by 
juvenile brown shrimp in Rincon Bayou were Spartina 
altemiflora, Spartina spartinae, detritus and benthic 



5-22 ^* Benthic Communities 



(a) Average macrofauna biomass (all stations) 



1000 



100 



3. 10 



O 



0.1 







\ 



Chlorophyll 
Macrofauna biomass 



\ ^ / 



E 

4 </> 
CD 

E 
o 



- 2 



1995 



1996 



1997 



1998 



1999 



2000 



(b) Average meiofauna abundance (all stations) 
1000 



100 - 



3 



10 - 



Chlorophyll 
Meiofauna abundance 




1995 



1996 



1997 



1998 



1999 



6000 



- 5000 



4000 E 

O 

c 

- 3000 m 

c 
cc 

T3 

2000 i 
< 

- 1000 



2000 



Figure 5-13: Comparison of marsh-wide average chlorophyll biomass with macrofauna biomass (a) and 
meiofauna abundance (b). 



Chapter Five ♦ 5-23 



diatoms (Riera et al. 2000). From 6''C and 6''N 
values, it appears organic matter inputs earned by river 
inflow could also contribute significandy to the feeding 
of migratory brown shrimp. In the marsh habitats of 
Rincon Bayou and the Nueces Delta, shrimp isotopic 
ratios changed rapidly, indicating high tissue turnover 
rates and rapid growth. Therefore, re-introduction of 
fresh water to the marsh results in conditions within 
nursery areas {e.g., higher benthic biomass) favorable 
for feeding and growth of juvenile brown shrimp. 
Further discussion of the approach and results of the 
stable isotope analyses conducted by Riera et al. (2000) 
has been included as Appendix E of this Concluding 
Report. 



Effects of Diversions as Disturbances 

Rapid changes in salinity due to flooding events can be 
classified as a disturbance. The frequency and timing 
of these freshwater inflow events into Rincon Bayou 
were very important. The effects of disturbance 
frequency and altered flow on macro-benthic 
community structure and colonization in Rincon 
Bayou were also independentiy studied during the 
demonstration period (Ritter and Montagna 2000). 
Abundance and biomass decreased with increasing 
disturbance frequency, indicating post-disturbance 
community persistence is important in regulating 
community structure. There was higher abundance 
and biomass in defaunated sediments relative to 
background sediments, indicating disturbance plays an 
important role in community production of early 
succession communities. The collection date was the 
most important factor determining community 
structure, thus natural variabilit)' overwhelmed effects 
of both the flow and disturbance frequency 
manipulations. The temporal changes were driven by a 
Strehlospio benedicti recruitment event (resulting in 
abundances as high as 1.3 10' m"^ captured June 20, 
1997 (one day before the beginning of Event 16) and 
during the subsequent freshwater event. After the 
event, S. benedicti abundance declined rapidly, and 
freshwater species invaded, leading to the progression 
of three distinct community states {i.e., community 
structure and species dominance changed three times) 
over the 14-week period of the study. The 



overwhelming significance of "temporality" {i.e., 
short-term temporal change in community structure) 
was the unexplained temporal component of 
commumty variation in experimental manipulations. 
Temporality is simply a smaller temporal scale than 
seasonality'. The importance of short term changes 
relative to flood events indicated that the 
demonstration project was responsible for high 
productivity during the summers of 1997 through 
1999. Further discussion of the approach and results 
of the stable isotope analyses conducted by Ritter and 
Montagna (2000) has been included as Appendix F of 
this Concluding Report. 



SUMMARY 

Benthos in Rincon Bayou are normally under great 
stress due to high salinity concentrations, especially 
during summer. The benthos responded positively to 
inflow events by increased biomass, abundances and 
diversity. Direct and indirect mechanisms were 
responsible for the benthic response. The direct 
mechanism included increased physiological tolerance 
to oligohaline and estuarine salinity concentrations 
relative to hyper-saHne conditions. Inflow events at 
various times during the year, but especially in the 
spring and fall, were likely cues for benthic 
reproduction and settiement of planktonic lan'ae. 
Trophic interactions and habitat utilization represented 
indirect mechanisms to which benthos responded. 
Increased microalgal food and marsh habitat 
availability stimulated secondary production. As the 
frequency and magnitude of freshwater inflow events 
increased due to the demonstration project, the 
opportunity for positive responses also increased. 
Without the demonstration project features, Rincon 
Bayou would experience considerably less inflow, and 
revert to a reverse estuary with consequentially less 
biodiversity and productivity. 



5-24 



Benthic Communities 



CHAPTER SIX 



KENNETH H. DUNTON 

Marine Science Institute 
University of Texas, Austin 

HEATHER D. ALEXANDER-MAHALA 

Marine Science Institute 
University of Texas, Austin 



Vegetation 
Communities 



"Ye marshes, how candid and simple and 
nothing withholding and free 

Ye publish yourselves to the sky and offer 
yourselves to the sea! 

Tolerant plains, that suffer the sea and the 
rains and the sun. . ." 

♦ Sidney Lanier (1878) 



INTRODUCTION 

A primary factor affecting the growth and distribution 
of coastal halophytes (salt-tolerant plants) is soil 
salinity (Chapman 1974; Ungar 1974; Riehl and Ungar 
1982; Clewell 1997). Halophytes are able to tolerate 
relatively high concentration of sodium (Na*) and 
chlorine (CI ) because of physiological mechanisms 
allowing them to exclude, compartmentalize or extrude 
salts (Badger and Ungar 1990). Several halophytes, 
including Salkomia sp. and Suaeda sp., even exhibit 
stimulated growth at some salinity levels (Ungar 1991). 
However, there is for each species a salimty 
concentration at which the effectiveness of these 
mechanisms is compromised, and growth and 
reproduction are limited (Adam 1990). 

High soil salinity negatively impacts the reproductive 
ability of halophytes by decreasing seed viability. 
Hypersalinity can result in a reduction in the number 
of seeds germinating, a delay in the initiation of 
germination and an increase in the number of seeds 
remaining dormant (Ungar 1962; Chapman 1974; 
PhilipupiUai and Ungar 1984; Ungar 1995). Each of 
these consequences ultimately leads to decreases in 
plant cover and increases in bare soils, which can 
persist indefinitely until fireshwater inimdation (via 
either precipitation or flooding) occurs diluting the 
soils, alleviating the salinity stress and breaking the 
osmotically induced seed dormancy (Ungar 1962; 
Ungar 1978; Ungar 1995). Once germination takes 
place, successful seedling growth is also salimty 
mediated. The period of seedlmg development is 
probably the most sensitive time during the life cycle 
of a halophyte because the seedlings develop close to 



Chapter Six ♦ 6-1 



the soil surface, exposing them to salinity levels 2 to 
100 times that of the subsoil (Ungar 1978). 

While adult plants have been reported to tolerate 
salinity levels 10 to 100 times greater than seedlings 
(Mayer and Poljakoff-Mayber 1963), high soil salinity 
levels can negatively affect adult plant growth as well. 
Plant sunaval does not necessarily mean plant growth, 
as a plant may continue to sun-ive at a particular 
salinity level without increasing in si2e or actively 
reproducing. Several primary mechanisms have been 
suggested to explain the negative effects of 
hypersalinity on halophytes. These include ion toxicity 
of internal cells, interference with the uptake of 
essential nutrient ions, lowered external water potential 
and energy constraints (e.g., a large amount of energy is 
required to actively salt ions) (Greenway and Munns 
1983; Yeo 1983). 

Most halophytes can survive over a range of salinity 
concentrations, but no species has been reported to 
have maximal growth rates at salinity levels at or above 
seawater concentration (35 parts-per-thousand (ppt)) 
(Ungar 1991). For example, Spartina foliosa, a California 
salt marsh plant, was found to have 50% less dry mass 
production in sea water than in fresh water, with only 
39% of the plants sur%dving in the saltwater treatment 
(Phleger 1971). Barbour (1970) reported growth 
reductions in Salicomia virginica and Distichlis spicata at 
salinity values ranging from 5 to 22 ppt. Adams (1963) 
noted that in North Carolina salt marsh plants could 
not tolerate soil salinity levels over 70 ppt. Allison 
(1992) found a reduction in species number in a 
California salt marsh after periods of low freshwater 
availability suggesting that only a few stress -tolerant 
species could survive the high saUnity. 

In many instances, short-term freshwater flooding of 
hjrpersaUne marshes leads to an increase in primary 
productivity. Zedler (1983) found biomass oi Spartina 
foliosa to increase 40% in the Tijuana Estuary, 
CaUfomia, after two months of flooding rains. Covin 
and Zedler (1988) noted a 60% increase in the stem 
density oi S. foliosa after summer reservoir discharges 
and sewage spills along the Mexico border. They also 
found near extinctions of Salicomia higelovii and Suaeda 



esteroa during a drought period in 1984 that led to 
hypersaline conditions. 

In the Nueces Delta, hypersalinity occurs as a result of 
both natural and human-induced conditions. The 
region is semiarid, having low annual rainfall (70 
centimeters (cm) or 28 inches per year) and hot, dry 
summers. A net annual water deficit is common, as 
evaporation (1 52 cm or 60 inches per year) often 
exceeds precipitation (Longley 1994). These 
conditions produce hj^persaline soils that are diluted 
only through direct precipitation or by flooding of the 
Nueces River. The natural salinity stress is accentuated 
in the Nueces Delta because considerable harnessing of 
river water for municipal, agricultural and industrial 
purposes has reduced the opportunity for freshwater 
flooding events into the marshlands (Irlbeck and Ward 
2000). In years prior to the demonstration project, the 
river breached its banks and flooded the marsh only 
during infrequent flooding events. 



OBJECTIVES 

1) To determine the effects of the demonstration 
project on the open water and pore water salinity 
and nitrogen levels; and 

2) To determine the project effects on the 
distribution and abundance of emergent marsh 
vegetation at three different stations over four 
growing seasons in the upper Nueces Delta. 



MATERIALS AND METHODS 

Monitoring Stations 

The emergent vegetation and related physio-chemical 
parameters (i.e., salinity and nitrogen levels) were 
quantified at three sampling stations in the upper 
Nueces Delta, including one reference station and two 
treatment stations (Figure 6-1). Station I (Reference 
Station) was located west of the tidal flats area of the 
upper delta, about 0.9 km from the outfall of the 
Rincon Overflow Channel. This location was selected 
to limit the amount of influence by fresh water 
diverted by the demonstration project, but also to 



6-2 



Vegetation Communities 



Northern 
bluff line 



Approximate Scale 
Kilometers 




'^incon Bayou 



central Rincon Bayou Site 
(treatment) 



Figure 6-1: Location of vegetation sampling stations in the upper Nueces Delta. The study design included 
one reference site and two treatment sites (Tidal Flats and central Rincon Bayou), with one sampling station per site. 



allow to the same meteorological influences as the 
other two treatment stations. Station II was located in 
the tidal flats area about 1.2 km downstream from the 
Reference Station and about 0.5 km downstream from 
the Rincon Overflow Channel. Freshwater flow 
through the Rincon Overflow Channel direcdy 
impacted Station II. Station III was located adjacent to 
the central Rincon Bayou chamiel about 2.7 km 
downstream from Station II, 3.1 km from the Rincon 
Overflow Channel and 5.7 from the Nueces Overflow 
Channel. Station III was closest to Nueces Bay (about 
7.4 km via channels). Vegetation at Station III was 
potentially influenced by fresh water entering the 
marsh via the Nueces Overflow Channel and was 
subjected to tidal inundation of saline waters from the 
bay. 



The vegetation at the three stations was diverse, 
including both annual and perennial species. Annual 
plants differed from perermial plants in that they 
complete their life cycle within one year and reproduce 
by seeds (sexual reproduction). Perennial plants can 
live for more than a year and reproduce both sexually 
by seeds and by asexual vegetative growth {i.e., 
expansion of above-ground tissues and below-ground 
rhizomes). The dominant vegetation species at the 
three stations were the perennial plants Borrichia 
frutescens^ Bads maritima, Monanthocloe littoralis and 
Distichlis spicata. The perennial succulent Salkornia 
virginica was dominant at Station III but was rarely 
found at the odier two stations. The amiual succulent, 
Salkornia bigelovii, was periodically present at all three 



Chapter Six ♦ 6-3 



stations. The annual species Suaeda maritima, Lycium 
carolinianum^ and Umonium nashii were occasionally 
found at the stations. 



Open Water and Pore Water Chemistry 

Four replicate open water samples were collected from 
the water adjacent to the sampling transects. Two 
replicate sediment pore water samples were collected 
using lysimeters placed within the transect sediments at 
the 0, 49 and 99 m marks. Lysimeters were made of 
PVC pipes (6 cm diameter, 60 cm length) with 
horizontal slits cut in the lower 30 cm to allow for the 
passive movement of water into the pipe. Prior to 
sampling, lysimeters were pumped dry and aDowed to 
refill. If water were unavailable, sediment samples 
were taken and later centrifuged to extract pore water. 
After being centrifuged, salinity of pore water samples 
was determined using a refractometer. Oftentimes, the 
sediment was too dry to extract any water, and 
therefore data were not acquired on several sampling 
dates. In the field, open water sahnity was recorded 
witli an Orion conductivity meter and reported in ppt. 
AU samples were collected in botdes and placed on ice 
for later determination of ammonium (NH4*) and 
nitrite plus nitrate (NO, + NO3) levels. 
Concentrations of NH4*, and NO, + NO3 were 
determined using standard colorimetric techniques 
(Parsons ^/-a/. 1984). 

Freshwater flow through Rincon Bayou and direct 
precipitation were also measured during the 
demonstration period (Chapter 3). Daily data for these 
two freshwater variables were summed according to 
the periods tmmediately preceding each vegetation 
sampling date to allow comparison with salinity values. 



Transect Sampling 

Seasonal vegetation distribution and abundance were 
quantified using transect sampling (Bertness and 
Ellison 1987). Generally, sampling occurred in the late 
spring (~June), late summer (~September), late fall 
(~December) and mid-winter (~January). The 
sampling schedule was based on a previous study in the 



Nueces Delta, which suggested that the emergent 
vegetation exhibits peaks and declines in growth at 
different times of the year. Annual species were noted 
to increase in cover during the late spring but were 
non-existent throughout the summer, fall and winter. 
Additionally, many species exhibited a reduction in 
cover during the late fall because of the shedding of 
their leaves. Late summer sampling was chosen 
because the plants typically experience several months 
of increased temperatures and decreased rain. Mid- 
winter sampling was selected because it is the time of 
year when plant cover is typically low, but annual 
species seedling growth has begun. 

In June 1995, three permanent transects were 
established, one at each station. The transects were 
99 m long and 8 m wide (792 m^ at the Reference 
Station and Station II (Figure 6-2), and 103 m long and 
8 m wide (824 m^ at Station III. At all three stations, 
the transect lines were spaced at 3 m inter\^als for the 
first 9 m of the transect. At the Reference Station and 
Station II, the lines were spaced at 10 m intervals 
between 9 and 99 m. Each transect extended 
perpendicularly from the vegetation line at the water's 
edge. The transect lines were spaced closest together 
near the water's edge because this was expected to be 
the part of the transect showing the greatest variation 
in degree of tidal inundation and soil moisture. 
Station III differed in transect size and sampling 
intervals because a small channel intersected the 
transect between the 47 m and 57 m marks, resulting in 
the occurrence of three water's edges at this station. 
The transect design for Station III was different from 
the Reference Station and Station II because there was 
no area large enough to encompass at least a 99 m 
transect direcdy along Rincon Bayou. However, the 
difference in transect design was inconsequential 
because the three stations were uniquely different, and 
direct statistical comparisons between the stations was 
not necessary. At Station III, the lines were spaced at 
10 m intervals between 9 and 39 m, 8 m between 
39 and 47 m, 10 m between 47 and 57 m, 3 m inten'^als 
between 57 and 63 m and 10 m inter\'als between 
63 and 103 m. The transects were sampled at 2 m 
intervals along the horizontal transect lines, for a total 



6-4 



Vegetation Communities 



Water's 
edge 



8m 



3m 



10m 



99 m 

Figure 6-2: The layout and dimensions of a typical vegetation transect. The figure illustrates the transect design 
used at the Reference Station and Station II. Each dot is a sampling point for vegetation parameters (i.e., percent cover 
and leaf area index). Asterisks (*) denote the sampling locations of pore water samples. 



of 65 sampling points at the Reference Station and 
Station II and 75 sampling points at Station III. 



Percent Cover and Leaf Area Index 

Percent cover and leaf area index (LAI) were sampled 
seasonally from June 1995 to December 1999, for a 
total of 20 seasonal sampling dates. The three stations 
were sampled one additional time in June 2000, but 
only data for percent cover were acquired. However, 
the Station III transect was partially destroyed by 
livestock, and therefore percent cover data for that 
date were not collected. 

A 0.25 m' quadrat subdivided into 100 cells was used 
to estimate the percent cover of each species at each 
sampUng point. CeUs with no vegetation and those 
covered with water or wrack (dead plant material) were 
considered bare area. LAI, a measure of plant foliage 
density and distribution, was quantified at each 
sampling point using a LAI-2000 Leaf Canopy 
Analyzer (LI-COR, Lincoln, Nebraska). LAI provided 
a non-destructive means of estimating foliage density 
by measuring the amount of light attenuated within the 
canopy. Each LAI measurement was an average of 
three individual readings taken at a sampling point. 
LAI readings for 1995 were not included in the 
analyses due to complications with the measuring 
technique at die beginning of die sampling period. 



Analyses 

Vegetation parameters were analyzed at two different 
spatial scales (792 or 824 m" transects and 0.25 m" 
quadrats interpolated to 0.25 m'grid cells). Large-scale 
analyses averaged data over an entire transect and 
utilized traditional methods, including graphs and 
tables. Small-scale analyses were accomplished 
through the use of a Geographical Information System 
(GIS). The GIS allowed each sampUng point and 
corresponding data to be geographically represented, 
taking into account the spatial relationships between 
vegetation parameters. Analysis at two different scales 
was necessary because much of the heterogeneity 
foiind at small scales was lost when the data were 
averaged over an entire transect. Large-scale 
obser\'ations were useful in that they indicated general 
patterns and trends in vegetation parameters, such as 
seasonal peaks and declines, but small-scale analyses 
provided detailed information regarding changes in 
vegetation species distribution and abundance. 

Large-Scale Analyses 

Total transect average percent cover for each species 
was determined by adding together the percent cover 
of each species at each sampling point and then 
dividing by the total number of sampling points. Total 
LAI was calculated in the same manner. These total 
ntimbers provided an estimate of species percent cover 
and LAI on a large-scale basis. 



Chapter Six •♦♦ 6-5 



Small-Scale Analyses 

Incli\'idual species percent cover and LAI readings 
taken at each sampling point were analyzed on a small- 
scale basis by the means of a GIS. Each sampling 
point was given geographic coordinates (Universal 
Transverse Mercator; Zone 14) based on differential 
Global Positioning System (GPS) points taken in the 
field at the four comers of the transects. Percent cover 
and LAI data acquired in the field were assigned to the 
corresponding geographic location at each sampUng 
point then incorporated from tabular format in 
Microsoft Excel to ArcView GIS software. The 
Inverse Distance Weighting function in Arc View's 
Spatial Analyst extension was used to interpolate grids 
for the area within the transect using the data points 
taken in the field as reference numbers. The method 
assumes that each point has a local influence that 
diminishes with distance so that points closer to the 
cell containing the field measurement have greater 
values than those farther away. A grid cell size of 
0.25 m (the same size as the quadrat) was used, 
providing a means of small-scale analyses. 

Maps of percent cover and LAI for each transect were 
analyzed to determine changes in species cover and 
bare area before and after hydrographic events at each 
station. For percent cover, vegetation maps for each 
sampling date were created by querying the data to find 
areas in the transect where cover for each individual 
species were greater than 50%. This allowed all species 
to be mapped together on the transect. Maps from the 
sampling date prior to an event were then compared to 
maps for the three sampling dates following the event. 
The conversion of the point data into interpolated 
surfaces allowed for mathematical manipulations and 
analyses in a spatially coherent, three-dimensional 
manner, which naturally represented the data. 



BlOMASS 

Four replicate samples of monospecific stands oi Batis 
maritima, Borrichia Jrutescens, Monanthocloe litioralis and 
Distkhlis spkata growing near the transect were sampled 
bi-annuaUy (winter and spring) using a PVC corer 
(10 cm diameter, 30 cm length). The corer was placed 



around the vegetation and above-ground plant material 
clipped. The corer was then dri\'en into the ground for 
collection of below-ground material. Plant material 
was sequentially sieved using a 1 cm sieve foUowed by 
a 1 miHtmeter (mm) sieve, sorted into above- and 
below-ground portions and dried at 60° C to a 
constant weight. Samples were weighed to the nearest 
0.1 gram (g), and biomass was converted to an area 
basis. Root to shoot (RjS ratios) ratios were calculated 
from biomass measurements. 



RESULTS 

Vegetation and physio-chemical parameters [i.e., 
salinity and nitrogen levels) were examined based on 
changes observed within and between the stations 
before and after periods with significant freshwater 
inflow or precipitation (Table 6-1). In general, 
vegetation from three different sampling periods 
exhibited measurable responses to these hydrographic 
influences: July 1997, October 1998 and September 
1999. The major hydrographic events (Chapter .3) 
preceding each of these sampling dates include Event 
16 (June 21 through July 3, 1997), Event 17 Quly 4 
through 26, 1997), Event 25 (October 16 through 29, 
1998), Event 35 (August 19 through September 3, 
1999) and Event 36 (September 4 through 20, 1999) 
(Table 6-1). Where any sampling period was preceded 
by more than one identified hydrographic event {e.g., 
July 1997 and September 1999), it was assumed that 
any measurable change observed in the vegetation 
could be due to a combination of those events. 
Therefore, for the purposes of this vegetation analysis, 
the events preceding each sampling period during 
which a response was observed were collectively 
referred to as the composite hydrographic events of 
July 1997, October 1998 and September 1999. During 
each of these periods, the Rincon Overflow Channel 
was activated and Station II was subjected to the 
influence of project diversions. 

Because salt marsh vegetation also often responds to 
direct precipitation, total monthly precipitation was 
reported separately to allow precipitation-mediated 



6-6 



Vegetation Communities 



Table 6-1: Hydrographic events (from Table 3-3) occurring prior to each vegetation sampling period. The three 
composite hydrographic events which stimulated a vegetative response (July 1997, October 1998 and September 1999) 
are indicated in bold. Cumulative flow, rainfall and average salinity concentrations also reported for all vegetation stations 
(1 through 3). Daily values for net flow and total precipitation were summed between sampling intervals. In some cases, 
sampling occurred during an event. The Nueces Overflow Channel was completed on October 26, 1995, so flow prior to 
that date was zero. Precipitation data for sampling dates prior to May 16, 1996 were that recorded at the Corpus Christi 
International Airport. Hyphens (-) indicate missing data. 



Vegetation 


Hydrographic Events 


Total Net Flow Through 


Total Precipitation 


Sampling Date 


(Table 3-3) 


Rincon Bayou (acre 


-ft) 


(inches) 


6/29/95 


1,2,3 







6.06 


8/25/95 


4 







4.84 


11/7/95 


5,6 


- 




14.57 


2/13/96 


7,8,9, 10 


- 




3.62 


5/22/96 




- 




1.64 


9/5/96 


11 


41 




9.37 


11/6/96 


12, 13, 14 


250 




3.26 


2/17/97 




79 




4.29 


6/2/97 


15 


57 




16.66 


8/29/97 


16,17 


1,542 




3.35 


11/26/97 


18 


264 




14.19 


1/5/98 




-1 




0.38 


6/1/98 


19 


78 




5.45 


10/2/98 


20,21,22,23,24 


670 




9.89 


12/2/98 


25, 26, 27 


3,381 




13.11 


1/13/99 




-8 




1.08 


6/2/99 


28, 29, 30, 31 


-1 




7.40 


9/2/99 


32, 33, 34, 35 


-41 




13.72 


12/8/99 


36,37 


820 




4.63 



changes to be identified (Figure 6-3). It should be 
noted that, although direct precipitation was not an 
effect of the demonstration project, it was a freshwater 
source to which vegetation responded, which could 
also be used to indicate the importance of freshwater 
diversions. 



Salinity 

open Water Salinity 

Open water salinity values for all three stations 
increased and decreased simultaneously throughout the 
study period, although the magnitude of change varied 
between the stations (Table 6-2 and Figure 6-4a). 
Decreases in salinity were often seen following 
precipitation or flooding, and high salinity values were 
common during dry, hot periods. An important 
assumption was that changes in open water and pore 



water salinity values at the Reference Station were not 
due to the channels, being determined by 
environmental conditions such as precipitation, 
evaporation and run-off. This was important when 
comparing the salinity values between the stations and 
assessing the relative impacts of the channels. 

When evaluating salinity levels, precipitation and flow 
effects were analyzed independendy because changes in 
salinity could be due to one or both parameters 
(Table 6-1 and Figure 6-4a). The data acquired in this 
study indicated that heavy precipitation could occur 
independent of flow and result in low salinit)' values. 

Therefore, when analyzing the salinity data, sampling 
dates that experienced prior hea\7 precipitation 
without heavy flow were treated separately from all 
other events (Figure 6-5). When interpreted in this 
manner, there was a positive correlation between 
increasing flow and decreasing salinity values at both 



Chapter Six ♦ 6-7 



10 
9 
8 



0) 



oj 5 



I— 
o 



4 
3 
2 - 

1 T 

-L 



T ~ 

1996 



J 



la 



25 



20 



15 -Si 

1 

c 

S 

10 TO 
O 



- 5 



1997 



1998 



1999 



2000 



Figure 6-3: Total monthly precipitation during the demonstration period. Data from April 1995 
through April 1996 are that reported from the Corpus Christi International Airport, and data from 
May 19996 through December 1999 are that recorded at the Rincon gauge. 



Table 6-2: Mean open water (OW) and pore water (PW) salinity values at each station. Pore water samples taken 
at three locations (0, 49 and 99 m) along each transect. Hyphens (-) indicate missing samples because of dry open 
water channels or pore water holes. Each value reported reflects the average of two individual samples. 









Reference Station 






Station II 






Station III 




Yr 


Mo 


OW 


PW 




OW 


PW 




OW 


PW 
Dm 49 m 






Dm 49 m 


99 m 


Dm 49 m 


99 m 


99 m 




Apr 


14.8 


20 


- 


23 


36 


- 


30 


32 


- 


95 


Jun 


- 


- 


- 


56 


50 65 


69 


58 


- 


- 




Aug 


10 


18 53 


56 


26 


43 68 


66 


28 


51 


70 




Nov 


4 


29 51 


36 


9 


30 70 


47 


23 


38 33 


36 




Feb 


27 


- 


- 


- 


70 


68 


47 


- 


- 


96 


May 


- 


- 


- 


- 


- 


- 


57 


- 


- 




Sep 


5 


44 68 


74 


5 


28 79 


- 


28 


- 


62 




Nov 


49 


46 64 


66 


58 


69 70 


71 


54 


63 


71 




Feb 


53 


49 


- 


65 


59 76 


61 


47 


- 


- 


97 


Jun 


13 


27 


- 


12 


13 70 


- 


21 


- 


- 




Aug 


43 


- 


- 


- 


- 


- 


22 


- 


- 




Nov 


12 


26 61 


52 


8 


8 36 


54 


15 


- 


40 




Jan 


14 


- 


- 


13 


- 


- 


18 


- 


- 


98 


Jun 


45 


- 


- 


- 


- 


- 


49 


- 


- 




Oct 


30 


46 81 


- 


33 


40 48 


62 


22 


52 41 


73 




Dec 


12 


37 55 


- 


11 


34 53 


69 


16 


- 


- 




Jan 


18 


- 


- 


15 


- 


- 


18 


- 


- 


99 


Jun 


23 


30 72 


- 


47 


54 80 


83 


44 


54 51 


81 




Sep 


6 


23 83 


42 


7 


18 21 


41 


12 


- 


11 




Dec 


17 


29 82 


87 


29 


60 85 


82 


29 


46 51 


92 



6-8 



Vegetation Communities 



50 



40 



I 30 



•S 20 



10 - 



(a) Open water salinity 
5000 




1996 



1997 



1998 



1999 



Total rainfall 

Total flow into Rincon Bayou 



■ ▼• ■ Average salinity; 
-o— Average salinity: 
-D— Average salinity: 



Reference Site (Station I) 
Station II 
Station III 



2000 



50 



40 



§ 30 



•i 20 



a 



10 



(b) Pore water salinity (at 49 m along the transect) 

5000 



4000 



CD 3000 
o 

I 2000 



1000 



H 



^y^-^i 



I 




140 
1- 120 



100 c:- 

Q. 



80 
60 
40 
20 




Q. 
>. 

To 
CO 



1996 



1997 



1998 



1999 



2000 



I I Total rainfall 

Total flow into Rincon Bayou 



• ▼• • Average salinity: 
-o— Average salinity: 
-a — Average salinity: 



Reference Site (Station I) 
Station II 
Station III 



Figure 6-4: Salinity for open water (a) and pore water (b) at each sampling site for each sampling date. Cumulative 
daily rainfall and inflow into Rincon Bayou also plotted for each period between sampling dates. 



Chapter Six ♦ 6-9 



(a) Reference Station 



l\J 












60 












5-. 50 

Q. 

|40 


• 
• 


• 




• 




CO 
CO 

20 
10 


• 
o 


o 


• 
• 


~~~~" ~~~- 


._____j^ = 0.33 





oo 








1 1 1 1 1 



(b) Station II 



1 

60 


• 


• 








_ 50 

Q. 

B 40 ■ 
1 30 


• 




• 


^""■^----.^..^^^^r^ = 68 




'«20. 








^^^^^^ 




10 





o 






^^-^---..•^ 



(c) Station III 



l\J ■ 














60 














_50 

Q. 

3 40 


t 


• 










alinity 

GO 

o 


o 




• 




- r2 = 56 




"20 
10 


o 


o 


• 


• 


^^^^-— 


_____^ • 


J 















500 1000 1500 2000 2500 3000 3500 4000 4500 5000 

Flow Into RIncon Bayou (acre-ft) 



Figure 6-5: Correlation between total flow through RIncon Bayou and open water salinity for each 
station. Open circles (O) indicate salinity values on sampling dates following heavy precipitation (> 25 cm) 
with little to no flow (< 617 10^ m^ or 500 acre-ft). Closed circles (•) indicate salinity values on all other 
sampling dates. The separation of salinity values was used to distinguish the effects of freshwater inflow. 
The linear regressions indicate the correlation between flow and salinity values only for events with large 
inflow. As flow increased, salinity decreased at the treatment stations (Stations II and III). As expected, a 
clear correlation was not found at the Reference Station. 



6-10 



Vegetation Communities 



Station II (r = 0.68) and Station III (r = 0.56) and a 
nonsignificant correlation at the Reference Station 
(r^ = 0.33). The correlations were based the total 
precipitation and flow experienced at the stations 
between sampling periods. However, on two 
occasions, sampling dates were only two months apart 
(November 1997/January 1998 and December 
1998/January 1999). For these two periods, flow and 
precipitation for the three months prior to the 
sampUng date were considered. 

At the Reference Station, salinity values greater than 
35 ppt (seawater concentration) were recorded 22% of 
the time, while values less than 18 ppt were measured 
56% of the time. The highest salinity (53 ppt) 
occurred in February 1997 during a five-month period 
with less than 25.2 cm (10 inches) of rain. The lowest 
values recorded coincide with periods having heavy 
rainfall. For example, a salinity value of 13 ppt was 
recorded in June 1997, following three months with 
over 37.8 cm (15 inches) of rain. However, salinity 
increased rapidly afterwards to 43 ppt in August 1997. 
A salinity value of 12 ppt was recorded in December 
1998, following four months with over 55.4 cm 
(22 inches) of rain. In this instance, salinity remained 
relatively low throughout 1999, most likely because 
spring rains were abundant, with the highest salinity for 
the year (23 ppt) being recorded in June 1999. 

At Station II, open water salinity values above 35 ppt 
were recorded on three sampling dates (25% of the 
samples), with the highest recording being 65 ppt in 
February 1997. Low values recorded at this station 
occurred on the same dates as those at the Reference 
Station. Values after major hydrographic events 
flooding the Rincon Overflow Channel do not differ 
by more than 1 ppt from those at the Reference 
Station but were 5 ppt lower than Station III after the 
October 1998 and September 1999 events. 

Station III had the greatest number of readings higher 
than 35 ppt (35%). The relatively higher values may be 
reflective of its closer proximity to Nueces Bay. The 
highest value (57 ppt) was recorded in May 1996, 
following six months of less than 5 cm (2 inches) total 
rainfall. 



Pore Water Salinity 

Pore water salinity values were typically higher than 
those in open water (Table 6-2 and Figure 6-4b). At 
the Reference Station, pore water salinity 
concentrations ranged between 20 and 87 ppt and were 
as much as 14 times higher than the open water salinity 
value measured on the same date. At Station II, values 
ranged between 8 and 85 ppt and were as much as 16 
times higher than the open water value. The range of 
values at Station III was similar to the other stations 
(1 1 to 92 ppt) but was measured to be only as much as 
3 times higher than the open water value. 

Pore water values did not always decline after a 
hydrographic event, whether the event was 
precipitation- or flow-mediated (Figure 6-4b). 
However, as many of the values are missing, 
determination of actual decrease proved difficult. 
Additionally, sampling dates did not always 
immediately follow times of heavy precipitation or flow 
events. Consequentiy, salinity values might have 
decreased, but unless the sampling date occurred soon 
after the freshwater input, any pore water response 
might not have been measured. 

On the occasions that values could be recorded, the 
response was variable. In December 1998, following 
three months with over 25 cm (10 inches) rainfall, pore 
water salinity values at the Reference Station decreased 
by 9 and 26 ppt at two locations (0 and 49 m). 
However, values at Station II were only reduced at m 
(the location closest to open water) and only by 6 ppt, 
despite the flooding of the station during the October 

1998 event. In contrast, pore water salinity values 
decreased at Station II in September 1999 by 36, 58 
and 42 ppt at 0, 49 and 99 m locations, respectively. 
The decrease immediately followed the September 

1999 event, which had not yet activated the Rincon 
Overflow Channel but did see over 15 cm (6 inches) 
rain in one day. During this event, a tidal surge from 
the bay flooded the transects, flushing the soils and 
then retreating after the surge. In this instance, the 
sampHng date was only days following the event. The 
spring and summer months also had relatively high 



Chapter Six ♦ 6-11 



rainfall compared to the other sampling years. The 
range of values at the Reference Station following the 
same event decreased only at m by 6 ppt, but 
increased at 49 m by 2 ppt and 45 ppt at 99 m. 



Ammonium 

Open Water Ammonium 

Concentrations of NH4* at the Reference Station 
exhibited the greatest variation (0.9 to 14.7 jimoles ), 
with peak values occurring in September 1996 
(12.1 (imoles ) and June 1999 (14.7 ^moles ) 
(Table 6-3). The increase in September 1996 
coincided with a peak at Station II. The June 1999 
peak occurred during a period of 4 months with over 
30.2 cm (12 inches) of rain. 

At Station II, concentrations resembled those at 
Station III, except for a peak value of 9.4 |imoles in 
September 1996, 51.6 (omoles in January 1997 and two 



peaks of 6.4 ^imoles in December 1998 and January 
1999. ITie first peak occurred during a period with 
more than 22.7 cm (9 inches) of rain in two months, 
and the last two peaks followed the October 1998 
event, which flooded the station. Values prior to the 
event were in the average range for that time of year 
(2.7 (imoles), but were more than t«.ice as high 
following the event. The concentration decreased by 
60% in the following five months to near 4.0 ^unoles, 
which represented the fall peak range under non-event 
conditions. 

At Station III, concentrations cycled over the study 
period, with low values (~2.0 to 3.0 |jmoles) being 
obser\xd in the late fall and winter. Values increased 
from winter to spring and peaked in the late summer 
or fall (4.0 to 6.0 (jmoles). Station III exhibited the 
smallest range of values, 1.6 to 9.5 [ijnoles. 



Table 6-3: Mean open water ammonium (NH/) and nitrite+nitrate (NO2 -•- NO3') concentrations at each station. 

Values are reported ± SE. (n = 4). Samples acquired prior to September 1996 represent only one water sample and 
therefore do not contain SE. Dashes indicate times when samples could not be taken. 



Yr 


Mo 


Reference Station 
NH/ NO2 + NO3 


Station II 
NH/ NOj + NO3 


Station III 
NH/ NO2 + NO3 




Apr 


3.7 


0.8 


5.5 


5.3 


2.5 


2,7 




Jun 


4.1 


1.2 


3.0 


20.2 


5.6 


2.3 


95 


Aug 


1.7 


1.3 


5.4 


1.3 


2.9 


5.6 




Nov 


3.1 


1.7 


1.7 


7.1 


3,0 


2.7 




Feb 


5,0 


1.8 


- 


- 


1,6 


1.1 


96 


May 


- 


- 


- 


- 


2,7 


0.7 




Sep 


12.1 ±1.00 


0.9 ± 0.05 


9.4 ±1.57 


1.2 ±0.08 


3,8 ±0,12 


1,8 ±0,5 




Nov 


1.9±0.12 


1.2 ±0.09 


1.6±0.11 


0.9 ±0.05 


2,3 ±0,12 


1,0 ±0,07 




Feb 


1.7±0.15 


0.8 ±0.01 


1.6±0.14 


0.9 ± 0.05 


1.7±0,15 


0,9 ±0,04 


97 


Jun 


2.3 ±0.05 


0.5 ±0.03 


3.2 ± 0.20 


0.7 ±0.04 


2,6 ± 0.08 


0,8 ±0,05 




Aug 


3.5 ±0.17 


0.8 ±0.08 


- 


- 


3.9 ±0,38 


0,8 ±0.03 




Nov 


2.6 ±0.15 


0.5 ±0.10 


3.7 ±0.17 


0.3 ±0.01 


4,1 ±0,14 


0,4 ± 0,02 




Jan 


3.9 ±0.07 


0.5±0.11 


51 .6 ±2.22 


1.9±0,18 


2,9±0,15 


0,7 ±0,17 


98 


Jun 


0.9 ±0.18 


0.8 ± 0.03 


- 


- 


1.7±0,11 


0,8 ±0,02 




Oct 


5.7 ±0.17 


0.9 ± 0.02 


2.7 ±0.09 


0,5 ±0.03 


2,4 ±0,10 


0,5 ±0.01 




Dec 


2.6 ±0.37 


0.7 ±0.02 


6.4 ±0.44 


1.9 ±0,09 


2,1 ±0,03 


0.9 ±0,03 




Jan 


2.5±0.19 


0.9 ±0.01 


6.4 ±1.50 


1.1 ±0,03 


2,3 ±0,05 


1.0 ±0,01 


99 


Jun 


14.7 ±0.67 


0.9 ±0.04 


3.8 ±0.27 


1,1 ±0,06 


5.0 ±0,23 


1.2 ±0,02 




Sep 


2.6 ±0.06 


0.4 ±0.02 


2.8 ±0.08 


0.5 ±0.01 


2,8 ±0,39 


0,5 ±0,03 




Dec 


8.0 ± 0.32 


0.9 ± 0.03 


4,6 ±0.10 


1.5 ±0.09 


9,5 ± 0,24 


1,1 ±0,05 



6-12 



Vegetation Communities 



Pore Water Ammonium 



NiTRITE+NlTRATE 



Pore water NH4* concentrations were generally much 
higher than open water levels, with the highest values 
recorded being in the summer and lowest values in the 
late fall and winter (Table 6-4). At the Reference 
Station, values ranged from 1.9 to 193 (xmoles. The 
highest values were recorded in June 1999 (53 to 
193 (xmoles). At Station II, values ranged between 
4.5 and 197 |xmoles. High values (64 to 197 |jjnoles) 
were measured over a six-month period from |une to 
December 1999 and coincided with a fourteen-month 
period with over 125.9 cm (50 inches) of precipitation. 
The highest values recorded at Station III were also in 
June through December 1999 (21 to 213 i^moles). At 
all three stations, values appeared to gradually increase 
over the study period (Figure 6-6), with similarly high 
values seen during the second half of 1999. 



Open Water Nitrite+Nitrate 

At the Reference Station, NO, + NO, concentrations 
exhibited the smallest range of values (0.4 to 
1.8 pmoles), with the highest value being recorded in 
February 1996 (Table 6-3). The highest overall values 
were seen at Station III during the first sampling year 
(1995), with values exceeding 2.3 jimoles at each 
sampling date during that year and approaching 
6 ^imoles in August 1995. The range of values for the 
remainder of the study period was between 0.4 and 
1.2 ^imoles. The values at Station II were between 
0.3 ^imoles and 5.3 ^moles, with the highest values 
recorded in April 1995. An additional small peak 
occurred in December 1998 (1.9 jimoles). 



Table 6-4: Mean pore water ammonium (NH/) and nitrite+nitrate (NOj" + NO3) concentrations at each station. 

Pore water samples were taken at three lysimeters located at 0, 49 and 99 m from the water's edge. Two water 
samples were taken at each lysimeter, and the two values averaged to determine a mean value for that distance. 
The range of mean values at the three distances is reported. If only one number is listed, then samples were only 
taken from one well, and dashes represent samples that could not be taken (as a result of sediments being too dry to 
extract water). 



Yr 


Mo 


Reference Station 
NH4* NOj + NO3 


Station II 
NH4* NO2 + NO3 


Station III 
NH/ NOj + NO3- 




Apr 


11.4 


3.5 


28.8 


1.4 


17.7 


0.5 




Jun 


2.4 to 21.8 


8.4 to 38.2 


37 to 48.5 


2.7 to 14.4 


- 


- 


95 


Aug 


4.9 to 28.4 


2.0 to 3.3 


16.2 to 53.7 


1.5 to 6.0 


7.4 to 12.9 


2.8 to 11.9 




Nov 


1 .9 to 1 1 .4 


2.7 to 12.1 


4.9 to 22.9 


0.1 to 4.8 


2.0 to 13 


0.2 to 1.0 




Feb 


8.6 


- 


6.1 to 13.2 


- 


- 


- 


96 


May 


- 


- 


- 


- 


- 


- 




Sep 


4.5 to 23.5 


2.4 to 31.3 


6.4 to 125.4 


4.1 to 49.8 


3.4 


0.5 to 0.7 




Nov 


10.9 to 66.6 


1,6 to 8.9 


24.7 to 36.8 


0.8 to 1.2 


7.8 to 9.3 


0.4 to 0.8 




Feb 


10.7 


1.3 


4,5 


25.1 


- 


- 


97 


Jun 


48.3 


0.5 


5.6 to 15.3 


0.8 to 1.0 


- 


- 




Aug 


- 


- 


- 


- 


- 


- 




Nov 


6.7 to 8 


0.9 to 3.6 


12.2 to 17.6 


1.7 to 2.6 


7.9 


2.9 




Jan 


- 


- 


- 


- 


- 


- 


98 


Jun 


- 


- 


- 


- 


- 


- 




Oct 


19.9 to 38.2 


0.9 to 4 


32.8 to 131.7 


0.6 to 4.7 


10 to 65.9 


0.5 to 5.0 




Dec 


10.6 to 45.6 


1.2 to 10.3 


48.7 to 64.7 


1 to 2.8 


- 


- 




Jan 


27.2 


2.3 


30.7-60.5 


1.0 to 2.0 


- 


- 


99 


Jun 


52.9 to 107.4 


- 


63.7 to 182.8 


1.3 


79.9 to 111.4 


- 




Sep 


26.8 to 79.7 


0,5 to 1 


67.1 to 106.3 


0.1 to 0.8 


21.1 to 55.5 


0.7 




Dec 


58.4 to 84.5 


- 


85.6 to 113.1 


- 


86.1 to 94,8 


- 



ChapterSix ♦ 6-13 



(a) Reference Station 



120 . 



100 - 



^ 80 
o 

I. 60 

+ 

S 40 \ 



20 





, T , ■ , 



—I — I — 1 — I — I — I — r- 



—\ — I — I — I — I — I — I — I — 1 — I — I — I — I — I — I — 1 — I — I — 1 — I — I — I — I — r- 



(b) Station II 

120 




T — I — 1 — I — 1 — 1 — I — I — I — I — r— 1 — I — I — I — I — I — I — 1 — I — I— 



— 1 — I — I — 1 — I 



(c) Station III 

120 




AJAODFAJAODFAJAODFAJAODFAJAOD 
95 96 97 98 99 

Figure 6-6: Mean pore water ammonium values for each station. Pore water samples were taken at thiree 
lysimeters located at 0, 49 and 99 m from the water's edge. Curves represent the best fit curve for the values taken at 
the 49 m location. The values reported for each distance is the mean of two samples. Occasionally, one or more 
locations were dry, so some sampling dates do not have measurements or have only one or two measurements. 



6-14 *♦* Vegetation Communities 



Pore Water Nitrite+ Nitrate 

Pore water NO," + NO3 concentrations were typically 

higher than open water values (Table 6-4). At the 

Reference Station, values ranged from 0.5 to 

38.2 (jmoles, and at Stations II and III, concentrations 

ranged from 0.1 to 49.8 |jinoles and 0.4 to 11.9 ^xmoles, 

respectively. 



large-scale whole transect analyses: 
Individual Species Responses to Events 

There was considerable variability in the percent cover 
of each species within and between stations before and 
after composite hydrographic events (Figures 6-7 
through 6-9). At the Reference Station and Station III, 
vegetation changes appear to be predominantly 
precipitation-mediated rather than flow generated, 
while changes at Station II occurred as a result of both 
flow through the channels and direct precipitation. 

Reference Starion 

Batis maritima - At the Reference Station, total 
transect percent cover of Batis maritima cycled 
seasonally with peaks generally occurring in the 
summer and declines in the late fall/early winter 
(Figure 6-7a). The greatest increase in cover (3.5% to 
23%) occurred between February and August 1997. 
Between February and June, the study area received 
52.1 cm (20.7 inches) of rain, or 57% of the total yearly 
rainfall. At the Reference Station, B. maritima exhibited 
a decrease in cover in June 1998. From April to June 
1998, there was litde precipitation (< 0.91 cm, or 
0.36 inches), unlike other springs during the study 
period when several inches of rain fell during the same 
period. 

Borrichia frutescens - Percent cover of Borrichia 
frutescens at the Reference Station exhibited an inverse 
correlation with Batis maritima cover (r^=0.61) 
(Figure 6-7b). Generally, peaks in B. frutescens occurred 
when B. maritima cover was relatively low. 

Distichlis spicata - At the Reference Station, cover of 
Disticblis spicata exhibited an overall decrease from 



10% in February 1996 to < 1% in June 1997 

(Figure 6-7c). Cover remained < 1% for the remaining 

study period. 

Monanthodoe littoralis - At the Reference Station, 
Monanthocloe littoralis cover remained relatively constant 
throughout the study period (29 to 42%), exclusive of a 
23% decline in cover (from 37 to 14%) between 
January and September 1999 (Figure 6-7d). The initial 
decline in cover occurred followed six months with 
about 45 cm (18 inches) rain. The lowest cover 
measured coincided with the passing of Hurricane Bret 
(August 1999). The storm released over 15 cm 
(6 inches) of rain in one day. The transect was also 
flooded by a tidal surge. M. littoralis cover increased 
20% (14 to 34%) in the two months following the 
hurricane, a period with litde to no rainfall. Similar, 
but smaller-scale declines in cover were seen following 
two or more consecutive months with several inches of 
rainfall (August to September 1996 and February to 
June 1997). The decreases in cover were always quickly 
followed by an increase in growth. 

Salicornia bigelovii - At the Reference Station, 
Salicomia bigelovii cover exhibited spring peaks greater 
than 10% cover occurring in 1996, 1997 and 1998 
(Figure 6-7e). The largest peak (30%) in June 1999 was 
about 20% higher than the other two peaks and 
occurred following a winter and spring with consistent 
rainfall (11 out of 14 months received over 5 cm 
(2 inches) of rain). 

Bare Area — On occasion, decreases in bare area 
corresponded with increases in Salicomia bigelovii cover. 
For example, S. bigelovii cover was about 29% in the 
summer 1999, and bare area cover decreased by about 
20% (Figures 6-7e and 6-7f). However, there was no 
strong correlation between the two parameters 
(r = 0.27 for S. bigelovii cover > 2%). The greatest 
bare area cover at the Reference Station (69%) 
occurred in September 1999 following the composite 
hydrographic event in September 1999, which included 
the landfall of Hurricane Bret. 



Chapter Six V 6-15 



(a) Batis maritima 



(d) Monanthocloe littoralis 



100 
80 
60 
40 
20 . 



i t i 




100 
80 
60 
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(e) Salicornia bigelovii 



100 



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100 
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(c) Distichlis spicata 



(f) Bare area 



100 
80 
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40 
20 



100 
80 
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JDJDJDJDJDJ JDJDJDJDJDJ 

96 97 98 99 00 96 97 98 99 00 



Figure 6-7: Reference Station average total transect percent cover for the five dominant species and bare area on 
each sampling date. Arrows indicate the dates of composite hydrographic events that caused measurable changes in the 
vegetation. Error bars represent ±SE (n=65). J = June and D = December. 



6-16 *♦* V^egetation Communities 



100 
80 
60 j 
40 
20 



(a) Batis maritima 



V V V 




100 
80 
60 
40 
20 



(d) Monanthocloe littoralis 



I I I I i I I I I I II I I I I I I I I I I I I I II II II I I I I I I I I I I II I I I I I I M I U I I I I I I I I I I I I II I I I I I I I IMI I I I I I I II I I I I I I I II I I I I I Ml II I M II I II III 



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(c) Distichlis spicata 



(f) Bare Area 



100 



80 



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100 



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1 1 1 1 1 1 1 1 I I I I 1 1 1 1 1 u 1 1 1 II II II III 1 1 1 1 M 1 1 1 I I 1 1 1 1 1 I I 1 1 1 H I 1 1 II I I I 1 1 1 1 1 1 1 1 1 II I M 1 1 III I 

JDJDJDJDJDJ JDJDJDJDJDJ 

96 97 98 99 00 96 97 98 99 00 



Figure 6-8: Station II average total transect percent cover for the five dominant species and bare area on each 
sampling date. Arrows indicate the dates of composite hydrographic events that caused measurable changes in the 
vegetation. Error bars represent ±SE (n=65). J = June and D = December. 



Chapter Six ♦ 6-17 



100* 


a) Batis maritima 




80 


1 


1 1 


60 






40 


A 




20 



1 1 1 1 1 1 1 1 1 1 1 1 M 1 1 [ 1 1 1 1 1 1 I [ 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 i 



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100 
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80 
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40 



(c) Salicornia virginica 




100 
80 
60 
40 
20 



(f) Bare Area 




JDJDJDJDJD JDJDJDJDJD 

96 97 98 99 96 97 98 99 



Figure 6-9: Station III average total transect percent cover for the five dominant species and bare area on each 
sampling date. Arrows indicate the dates of composite hydrograplilc events that caused measurable changes in the 
vegetation. Error bars represent ±SE {n=75). J = June and D = December. 



6-18 *♦* Vegetation Communities 



Station II 

Batis maritima — Batis maritima percent cover at 
Station II exhibited a different pattern than the other 
two stations (Figure 6-8a). Cover of this species 
peaked at 16% in November 1995 but then decreased 
by 11% from November 1995 to May 1996. The 
decrease began following three months (August 1995 
through November 1995) during which about 42.8 cm 
(17 inches) of rain fell. The drop in percent cover was 
Kkely the result of direct precipitation, since similar but 
smaller scale declines were seen at the other two 
stations during this time. On November 7, 1995, all 
three transects were flooded with 15.1 to 25.2 cm (6 to 
10 inches) of water as a result of about 25.2 cm 
(10 inches) of rain that fell in one day (October 26, 
1995) approximately two weeks earlier. 

Batis maritima cover increased 9.5% from May to 
November 1996. During this rime, a positive net flow 
of over 308 10' m' (250 acre-ft) passed dirough 
Nueces Overflow Channel (Events 11 through 14). 
However, the diverted water was derived from 
primarily small exchanges, and the Rincon Overflow 
Channel was not activated. Furthermore, the majority 
of the flow was during late October, which was not 
sufficient to account for significant changes in percent 
cover. Therefore, the observed increase was more 
likely correlated to precipitation which occurred during 
the late summer (i?.^., > 22.7 cm (9 inches) fell during 
August and September). Cover remained near 
constant for close to a year, then declined by 9.5% 
firom August 1997 to June 1998. The decline occurred 
immediately following the July 1997 event. During the 
period between the June and August sampling dates, a 
total of 9.0 cm (3.55 inches) of rain fell and a positive 
net flow of 1,902 10' m' (1,542 acre-ft) passed into 
Rincon Bayou. During Event 16, Rincon Overflow 
Channel was activated, and it was suspected that this 
was also the case with Event 17 (Chapter 3). 

Afterwards, cover leveled off and remained relatively 
stable, until it began to gradually increase in December 
1999 near the end of the study period. It should be 
noted that the major hydrographic events in October 
1998 and the precipitation direcdy falling on the station 
during the spring and summer of 1999 (> 35 cm, or 



14 inches) had no effect on cover. This was most 
likely a result of cover already being relatively low. An 
increase in cover began following the September 1999 
event, which deposited 15 cm (6 inches) of rain in one 
day and had a positive net inflow through the Nueces 
Overflow Channel of 1,012 10' m' (820 acre-ft) 
between the September and December sampling dates 
in 1999. During this period. Event 36 was also 
suspected to have activated the Rincon Overflow 
Channel (Chapter 3). 

Borrichia frutescens - A correlation between Borricbia 
frutescens and Batis maritima was not seen at Station II 
(r^ = 0.16). At Station II, B. frutescens cover declined 
slighdy (5%) from the beginning of the study period to 
the end (Figure 6-8b). 

Distichlis spicata - At Station II, Distichlis spicata 
cover decreased gradually from May 1996 (14%) to 
June 1997 (< 1%) and remained relatively low (2 to 
5%) for the remainder of the study period 
(Figure 6-8c). Cover fell to almost 0% following the 
October 1998 composite hydrographic event but 
increased in cover from 0% to 10% from December 
1998 to December 1999. 

Monanthodoe littoralis - At Station II, Monanthocloe 
littoralis cover remained relatively constant between 
June 1995 and November 1996 (about 20%) 
(Figure 6-8d). Cover declined sharply from November 
1996 (17%) to February 1997 (6%). Cover increased 
(12%) until December 1998 when it dropped 
continuously to 3% in September 1999. The decrease 
during the October 1998 event, and the lowest cover 
recorded (September 1999) and coincided with the 
flooding of the transect after a tidal surge which 
occurred due to the passing of Hurricane Bret. This 
decrease corresponded to decreases seen in M. littoralis 
cover at the Reference Station. Cover then increased 
by 5% in December 1999, after Event 36, which 
activated the Rincon Overflow Channel and flooded 
the transect with fresh water. 

Salicornia bigelovii — The most significant changes 
in percent cover were seen in the annual succulent 
Salicornia bigelovii At Station II, the species was 
basically non-existent until June 1998, when cover 



Chapter Six ♦ 6-19 



reached near 10% (Figure 6-8e). Cover decreased 
during the fall mondis foUowing the plant's annual life 
cycle (i.e., annual plants complete their life cycle within 
a year). Cover increased dramatically from < 1% to 
52% between January and June 1999. The increase in 
S. bigelovii occurred the summer following the October 
1998 composite hydrographic event [i.e., the period 
between the October 1998 and June 1999 sampling 
periods), during which a positive net flow of over 
4,160 10' m' (3,372 acre-ft) entered Rincon Bayou. 
The Rincon Overflow Channel was activated during 
Event 25, consequendy flooding the station. The 
increase in growth corresponded to a period where 
11 out of 14 months received over 5 cm (2 inches) of 
rain. 

The late fall 1998 event significandy lowered open 
water salinity values at Station II from about 45 ppt 
(June 1998) to about 11 ppt Pecember 1998). Open 
water salinity concentrations were also lowered by 
precipitation in the fall 1997 (October), which kept 
early winter salinity values below 15 ppt as well. The 
only spring without Salicomia bigelovii ^o^^jth was 1997, 
most likely because a fall event did not occur in 1996 
and winter salinity values were relatively high (46 to 
58 ppt) compared to the other years. 

An increase in cover at Station II during June 1999 
coincided with an increase at the Reference Station. 
However, the increase at Station II was almost twice 
that of the Reference Station. In June 1 998, the 
percent cover of i". bigelovii ^^s almost identical at the 
reference and Station II (Figures 6-7e and 6-8e). In 
this instance, late winter salinity concentrations were 
lowered by rainfall at both stations. Prior to June 
1999, winter salinity values at Station II were lowered 
as a function of freshwater flow through the channels, 
possibly explaining the almost doubling in S. bigelovii 
cover at Station II compared to the Reference Station 
that summer and the increase compared to the 
previous year. 



A similar increase in S. bigelovii cover was seen in the 
summer 2000. Cover in June 2000 at Station II was 
41%, while cover at the Reference Station was only 
12%. Once again, the differences are most likely a 
result of the hydrographic event that activated the 



Rincon Overflow Channel in September 1999 
(Event 36). The difference in cover between the June 
1999 and June 2000 sampling dates at Station II may 
be indicativ^e of the tuning of the flow event. The 
1999 event occurred in the late summer, while the 1998 
event occurred during the fall and was followed by 
several months of consistent precipitation. 

Bare Area - At Station II, increases (> 2 %) in 
Salicomia bigelovii cover corresponded to decreases in 
bare area (r^ = 0.64) (Figures 6-8e and 6-8f). During 
June 1999, S. bigelovii cover was about 50% and bare 
area decreased about 33%, the largest decrease in bare 
area obser\'ed during the study period. Bare area was 
highest (81%) in September 1999, which was similar to 
the peak found at the Reference Station (69%). 

Station III 

Batis maritima — Total transect percent cover of Baiif 
maritima exhibited a similar pattern to that seen at the 
Reference Station (Figure 6-9a). Cover cycled 
seasonally, and the greatest increase in cover (9.7% to 
37%) occurred between February and August 1997. In 
general, percent cover at Station III varied between 
10% and 37% and was greater than the other two 
stations. 

Borrichia frutescens - At Station III, percent total 
percent cover oi. Borrichia frutescens vincA between 10% 
and 25% (Figure 6-9b). No correlation was seen with 
Batis maritima {f — 0.05). Total cover oi B. frutescens 
declined gradually from February 1997 (25%) to near 
11% in December 1999. 

Salicomia virginica - Distichlis spicata was not a 
dominant species at Station III, so Salicomia virginica 
was analyzed instead because it occurred at Station III 
(Figure 6-9c). However, this species was rarely found 
at the other two stations. S. virginica cover was greatest 
in February 1996 (23%) but decreased to 12% in May 
1996 and continued to decline to 6.5% in August 1997. 
The low cover occurred following the July 1 997 event. 
Cover remained relatively low until October 1998, 
when it began to gradually increase to 16% in 
December 1999. The increase occurred after the 



6-20 



V^egetation Communities 



October 1998 event and continued throughout 1999, 
which had several months of consistent rainfall. 

Monanthocloe littoralis - At Station III, Monanthodoe 
littoralis cover remained relatively constant (13% to 
21%) over the study period (Figure 6-9d). Cover 
exhibited only minor decreases in cover in response to 
the July 1997 event (7% decline) and a 4% decrease 
after the October 1998 event. 

Salicornia bigelovii - At Station III, Salicomia higelovii 
was practically non-existent throughout the study 
period, with only 1% cover in June 1998 and 3% cover 
in June 1999 (Figure 6-9e). 

Bare Area — Bare area cover at Station III gradually 
increased 1 5% from the beginning to the end of the 
study period, but changes did not appear to be 
mediated by hydrographic events (Figure 6-9f). 



large-scale whole transect 
Analyses: Leaf Area Index 

Leaf area index , a non-destructive means of estimating 
total vegetation foliage density, exhibited considerable 
temporal and spatial variability within and between 
sampling transects. Measurements are reported as total 
transect values. Average LAI values for the transects 
exhibited seasonal peaks in the summer and declines in 
the fall and winter (Figure 6-10) and displayed a similar 
trend at all three stations. However, values at the three 
stations differed in range and magnitude. At the 
Reference Station, values range from 0.60 to 1.99 and 
had an average of 1.42 compared to a range of 0.89 to 
1.34 and an average of 1.13 at Station II. Values at 
Station III were higher than the other two stations, 
ranging from 1.64 to 2.43, with an average of 2.02. At 
the three stations, peaks in LAI occurred in September 
and November 1996, following a two-month period 
with over 22.5 cm (9 inches) of rain. LAI then 
declined in February 1997, which was a period with 
litde rainfall. Values increased and peaked in January 
1998, another period following a three-month period 
of over 30 cm (12 inches) rain. The third major peak 
shared by the three stations was in June 1999, which 
followed a four-month period with over 30 cm rain. 



This spring also followed the October 1998 event. 
The greatest decline in LAI occurred in September 
1999 and coincided with a large increase in bare area. 
The decline occurred several weeks after a tidal surge 
due to Hurricane Bret flooded the transects and 
drowned much of the vegetation. In general, large 
decreases in LAI coincided with increases in bare area. 



Small-Scale Analyses 

Percent Cover 

Changes in the percent cover of individual species were 
e\'ident during sampling periods before and after 
composite hydrographic events (Table 6-5). 

Spring Cover — Analysis of GIS maps created from 
springtime percent cover at the Reference Station 
indicated that bare area cover appeared to be greatest 
following fall periods with little precipitation 
independent of amount of spring precipitation 
(Figures 6-1 lb and 6-1 le). Salicomia bigelovii increases 
occurred when fall and spring precipitation were high 
(Figure 6- lid), and Monanthocloe littoralis cover and bare 
area decreased with increases in S. bigelovii. The maps 
indicate that S. bigelovii invaded previously bare areas. 

At Station II, maps indicate that bare area was greatest 
following fall periods with no flow events and/ or littie 
precipitation. In June 1997 (Figure 6- 12b), bare area 
was greatest compared to other springs. Prior to this 
spring, there was litde fall rain and no flow. May 1996 
and June 1998, springs foOowing falls with heavy rain 
but no flow event, had less bare area than June 1997 
but more than either June 1999 or June 2000. In the 
falls prior to June 1999 and 2000, there were 
freshwater diversions and heavy precipitation. 
Salicomia bigelovii cover was greatest in June 1999, 
having greater than 50% cover over 59% of the 
transect (Table 6-5). Cover was also high in June 2000, 
with 49% of the transect having greater than 50% 
cover. The maps show that S. bigelovii invasion 
occurred in previously bare areas. The maps further 
indicate that 'Qatis maritima cover decreased after 
flooding events (Figures 6- 12c and 6-1 2d). 



Chapter Six ♦ 6-21 



(a) Reference Station 



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98 



99 



Figure 6-10: Total transect leaf area index (LAI) for each sampling date at each transect. Error bars 
represent ± SE. J = June and D = December. 



6-22 *♦* Vegetation Communities 



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8 

<u 

1 






tis maritim 
a frutescei 
doe littora 
mia bigelo 
Bare An 


c 

E 
■2 


a> 0) % 
q.q: o 


c 

c 

a 






<o ;£ o o 

OQ o £ .9 
O 5 CO 


nj £ o o 
OQ o -c .u 

E £ "<5 


OQ 


Bonichi 

Monantho 

Salico 

Salico 


specie 

re area 
• to or fi 


U) 
0) 

o 






§ 1 '^ 




•ssg 


4) 














ummary 

pedes or 
□ dates p 


c 














a 






c 








J3 






o 

s 








V) Oi c 






<n 








Table 6-5: 

a particular 
from sampli 


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o 
u> 

n 
a 

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8 


L 




o 
o 

c 

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CH 


i 

1 

<n 


1 





Chapter Six ♦ 6-23 



(a) May 96 




(b) June 97 




(c) June 98 




(d) June 99 





tSRN Batis mantima 
ggg;^ Monanthocloe littorahs 
^3 Bomchia frutescens 
i Salicornia bigelovn 
Bare Area 



Figure 6-1 1 : Reference Station percent cover maps for the five springtime sampling periods. Shaded areas for 
each species represent parts of the transect covered by at least 50% of that species. Areas with no shading (white) were 
not covered by greater than 50% of any one species. Springtime cover differed dramatically over the years. May 1996 
represents cover following a fall with heavy precipitation (~36 cm) but a spring with little precipitation (-5 cm); June 1997 
represents fall with little precipitation (~9 cm) but a spring with heavy precipitation (~43 cm); June 1998 represents fail with 
heavy precipitation (-33 cm) but a spring with little precipitation (-5 cm); June 1999 represents fall with heavy precipitation 
(~36 cm) and a spring with heavy precipitation (~18 cm); and June 2000 represents fall with little precipitation (--5 cm) and 
a spring with little precipitation (~1 1 cm). Reference Station maps are shown for comparison because this site was not 
affected by flow diverted by the demonstration project 



6-24 



Vegetation Communities 



(a) May 96 






(d) June 99 




(e) June 2000 




F^ Batis maritma 
^^ Monanthocloe littorahs 
^^ Bornchia frutescens 
^la SalKornia bigelovii 
^M Bare Area 



Figure 6-12: Station II percent cover maps for the five springtime sampling periods. Shaded areas for each species 
represent parts of the transect covered by at least 50% of that species. Areas with no shading (white) were not covered by 
greater than 50% of any one species. Springtime cover differed dramatically over the years. May 1996 represents cover 
following a fall with heavy precipitation and no flow, June 1997 represents fall with little rain and no flow, June 1998 
represents mid-summer flow event and fall with heavy precipitation, June 1999 represents fall with flow event and heavy 
rains and June 2000 represents late summer flow event and early fall heavy precipitation. Note decreases in bare area 
and increase in Salicomia bigelovii cover in (d) and (e) following late summer and fall flow events. 



Chapter Six ♦ 6-25 




(a) May 96 




(b) June 97 




(c) June 98 




(d) June 99 



SSd Balis maritima 
^^ Monanlhocloe littoralis 
^S Bornchia frulescens 
\99^ Salicornia virginica 
^M Bare Area 



Figure 6-13: Station III percent cover maps for the five springtime sampling periods. Shaded areas for each 
species represent parts of the transect covered by at least 50% of that species. Areas with no shading (white) were not 
covered by greater than 50% of any one species. Springtime cover differed dramatically over the years. May 1996 
represents cover following a fall with heavy precipitation and no flow, June 1997 represents fall with little rain and no flow, 
June 1998 represents mid-summer flow event and fall with heavy precipitation and June 1999 represents fall with flow 
event and heavy rains. Note that Salicornia bigelovii cover is non-existent in the transect regardless of flow events. 



6-26 



}/egetation Communities 



cover maps indicates that both Batis maritima and 
Borrichia frutescens decreased in cover in June 1998 and 
1999 compared to other springs, while Monanthocloe 
littoralis, Salkomia virginica and bare area had increased. 
The most evident vegetation changes appeared have 
occurred in the front section of the transect, which was 
lower and subjected to more influence from freshwater 
diversions. The maps also indicate that M. littoralis was 
commonly found in the back section of the transect, an 
area that was rarely flooded. Furthermore, the greatest 
bare area was found in the back part of the transect, an 
area that remained relatively constant throughout the 
spring periods. 

July 1997 Composite Event - The July 1997 event 
resulted in few changes at the Reference Station 
(Figures 6-14a and 6-14d). Only 8.5 cm (3.4 inches) of 
rain fell in the study area between the June and August 
sampling period. The most obvious change was a 
temporary increase in Batis maritima cover from 1.7% in 
June to 12% in August (Table 6-5). The increase in B. 
maritima corresponded to a 4-fold decrease in Borrichia 
frutescens. However, B. maritima cover quickly dropped 
back to 1.5% in November 1997, and B. frutescens 
increased. Bare area also decreased slighdy by 5% 
between June and August but increased quickly 
afterwards. 

At Station II, the vegetation was inundated by water 
through the Rincon Overflow Channel and 
precipitation during the event. Following the event, 
Batis maritima cover increased 3.4% but decreased to its 
pre-event cover in November 1997 (Figures 6- 15a 
through 6-15c) (Table 6-5). Monanthocloe littoralis cover 
increased by ~7% in August but returned to its pre- 
flood values in November as well. The increase in 
these species direcdy following the event corresponded 
to an 11% decrease in bare area. Bare area also 
increased quickly thereafter. 

At Station III, Batis maritima cover increased 22% in 
August following the event but dropped dramatically 
by 34% from August to November (Table 6-5). The 
increase in B. maritima occurred in the front section of 
the transect, and corresponded to a decrease in 
Borrichia frutescens and a reduction in bare area to almost 
0% in that section of the transect (Figures 6- 16a and 



6- 16b). B. maritima cover retreated in November, and 
the B. frutescens zone near the back of the front section 
of the transect began to recover (Figure 6-16c). 

October 1998 Composite Event - At the Reference 
Station, few changes were seen in the transect during 
the two sampling periods following the event; 
however, major changes were seen the following spring 
(Figures 6-17a through 6-17d). GIS analyses indicate 
that in June 1999, Salicomia higelovii expansion into 
previously bare areas had occurred. The increase in 
S. higelovii also corresponded with a decrease in 
Monanthocloe littoralis and an increase in Batis maritima 
cover. 

During the October 1998 event, diversions through the 
Rincon Overflow Channel were significant enough to 
wash out the road crossing at the north end of the 
channel, inundating Station II with freshwater. In the 
sampling periods following the October 1998 event, 
several significant changes in vegetation percent cover 
occurred (Figures 6- 18a through 6-18d). Bare area 
decreased gradually from 86% in October 1998 to 14% 
in June 1999. The decrease in bare area began as an 
increase in Monanthocloe littoralis cover. However, by the 
following spring, Salicomia higelovii had occupied almost 
all previously bare area and several parts of the transect 
previously occupied by M. littoralis. 

Few changes were seen in the transect at Station III 
following the October 1998 event (Figure 6-19). 

September 1999 Composite Event - At the 

Reference Station, a major increase in bare area 
occurred following flooding of the transect due to the 
tidal surge of Hurricane Bret in August 1999 
(Figures 6- 19a and 6- 19b). Afterwards, 42.5 cm 
(17 inches) of rain fell between September and 
December, leading to noticeable vegetation increases 
and bare area decreases. Monanthocloe littoralis cover 
quickly filled in almost half of the bare area by 
December 1999 (Figure 6- 19c). Bare area continued to 
decrease in June 2000 and both Batis maritima and 
Salicomia higelovii covet increased (Figure 6-19d). 



Chester Six ♦ 6-27 




(a) June 97 




(b) August 97 




(c) November 97 




(d) January 98 



EiSJ Bal)s manl)ma 
W^ Monanlhocloe litoralis 



^^ Bornchia fnitescens 
i-^^ Sahcornia virginica 
^H Bare Area 



Figure 6-14: Reference Station percent cover maps on the sampling date prior and three sampling dates following 
the July 1997 composite hydrographic event. Shaded areas for each species represent parts of the transect covered 
by at least 50% of that species. Areas with no shading (white) were not covered by greater than 50% of any one species. 
Reference Station maps are shown for comparison because this site was not affected by flow diverted by the 
demonstration project. 



6-28 



Vegetation Commumties 



(a) June 97 




(b) August 97 




(c) November 97 




tSiS Batis mantima 
BS^ Monanthocloe littoralis 
^g Bornchia frutescens 
■■ Bare Area 



Figure 6-15: Station II percent cover maps on the sampling date prior and three sampling dates following the 
July 1997 composite hydrographic event. Shaded areas for each species represent parts of the transect covered by at 
least 50% of that species. Areas with no shading (white) were not covered by greater than 50% of any one species. 
During the event, the Rincon Overflow Channel was activated and Station II was inundated with freshwater. Note 
increases in Batis maritima and Monanthocloe littoralis and decreases in bare area in August 1997 following the event. 



Chapter Six ♦ 6-29 




(a) June 97 




(b) August 97 




(c) November 97 




(d) January 98 



ESSlJj Balis maritima 
Egg^ Monanlhocloe littoralis 
^^ Bornchia frutescens 
V//A Salicornia virginica 
■■ Bare Area 



Figure 6-16: Station III percent cover maps on the sampling date prior and three sampling dates following the 
July 1997 composite hydrographic event. Shaded areas for each species represent parts of the transect covered by at 
least 50% of that species. Areas with no shading (white) were not covered by greater than 50% of any one species. Note 
increases in Safe maritima in August 1997 following the event. Increases in 6. maritima corresponded to decreases in 
Bornchia fmtescens and decreases in bare area. 



6-30 



Vegetation Communities 




(a) October 98 



(b) December 98 



(c) January 99 





(d) June 99 



I ~l Bornchia frutescens 
^H Batis mantima 
■■ Salicorma tigelovii 
^M Monanthocloe littoralis 
^M Bare Area 




Figure 6-17: Reference Station percent cover maps on the sampling date prior and three sampling dates 
following the October 1998 composite hydrographic event. Shaded areas for each species represent parts of the 
transect covered by at least 50% of that species. Areas with no shading (white) were not covered by greater than 50% of 
any one species. Reference Station maps are shown for comparison because this site was not affected by flow diverted 
by the demonstration project. 



Chapter Six ♦ 6-31 



(a) October 98 




(c) January 99 




(d) June 99 




ISSI Batis mantima 

Monanthocloe Moralis 
Borrichia frulescens 
Salicornia bigelovn 
Bare Area 



Figure 6-18: Station II percent cover maps on the sampling date prior and three sampling dates following the 
October 1998 composite hydrographic event. Shaded areas for each species represent parts of the transect covered 
by at least 50% of that species. Areas with no shading (white) were not covered by greater than 50% of any one species. 
During the event, the Rincon Overflow Channel was activated and Station II was inundated with freshwater. Note the 
increase in Salicornia bigelovii cover and decreases in bare area in the spring (June 1999) following the event. 



6-32 



Vegetation Communities 




(a) October 98 




(b) December 98 




(d) June 99 




(e) September 99 




(f) December 99 



^3 Batis marihma 
ggg^ Monanthocloe lilloralis 
|===| Bornchia fnitescens 
I Salicornia virginica 
Bare Area 



Figure 6-19: Station III percent cover maps on the sampling date prior and three sampling dates following the 
October 1998 composite hydrographic event. Shaded areas for each species represent parts of the transect covered 
by at least 50% of that species. Areas with no shading (white) were not covered by greater than 50% of any one species. 
Little change was observed at Station III during this event. 



Chapter Six ♦ 6-33 



(a) June 99 




(b) September 99 




(c) December 99 




(d) June 2000 



^a Bat)s mantima 
^^ Monanthocloe littoralis 
^M Bornchia frutescens 
^^BBj Salicorma bigelovii 
^B Bare Area 




Figure 6-20: Reference Station percent cover maps on the sampling date prior and three sampling dates 
following the September 1999 composite hydrographic event. Shaded areas for each species represent parts of the 
transect covered by at least 50% of that species. Areas with no shading (white) were not covered by greater than 50% of 
any one species. Reference Station maps are shown for comparison because this site was not affected by flow diverted 
by the demonstration project. Note the increase in bare area in September 1999 following the tidal surge of Hurhcane 
Bret. 



6-34 



Vegetation Communities 



(a) June 99 




(b) September 99 




(c) December 99 




(d) June 2000 




^3 Batis mantima 
ES^ Monanlhocloe littoralis 
^g Bornchia frutescens 
^S Salicornia bigelovii 
^B Bare Area 



Figure 6-21: Station II percent cover maps on the sampling date prior and three sampling dates following the 
September 1999 composite hydrographic event. Shaded areas for each species represent parts of the transect 
covered by at least 50% of that species. Areas with no shading (white) were not covered by greater than 50% of any one 
species. During the event, the Rincon Overflow Channel was activated and Station II was inundated with freshwater. 
Note the increase in bare area in September 1999 following the tidal surge of Hurricane Bret. This was not a freshwater 
inundation effect. 



Chewier Six ♦ 6-35 




(a) January 99 




(b) June 99 





(d) December 99 



Ea3 Balis maritma 
^g Monanthocloe lilloralis 
^^ Borne hia frutescens 
K^d Salicornia virginica 
^H Bare Area 



Figure 6-22: Station III percent cover maps on the two sampling dates prior and two sampling dates following 
the September 1999 composite hydrographic event. Shaded areas for each species represent parts of the transect 
covered by at least 50% of that species. Areas with no shading (white) were not covered by greater than 50% of any one 
species. 



6-36 



Vegetation Communities 



The tidal surge in August produced similar decreases in 
vegetation cover at Station II (Figures 6-20a and 
6-20b). The transect was then flooded by fresh water 
through the Rincon Overflow Channel several days 
later. Bare area had decreased by ~11% in December 
1999, following the event. By June 2000, bare area had 
decreased by —40% and Salicomia bigelovii had invaded 
~50% of the transect in areas that were previously 
bare. 

In January 1999, the most significant change was the 
appearance of Salicomia virginica in the back section of 
the transect (Figure 6-21 c). However, the cover had 
decreased to levels below the 50% mapping level in 
June 1999. An increase in Batis maritima also occiured 
in the spring following the event (Figure 6-21d). No 
obvious vegetation changes occurred in the transect at 
Station III following the tidal surge or the freshwater 
flow through the channels during the September 1999 
event. 

Leaf Area Index 

Vegetation maps showing the LAI distribution within 
and between the transects indicate that overall fohage 
density varied considerably. At the Reference Station, 
LAI values were highest in January and June 1998 and 
June and December 1999 (Table 6-6). The high 
LAI values appear to coincide with high Monanthodoe 
littoralis cover (Figures 6-14d and 6-23e). M. littoralis 
also appears to be responsible for high LAI values in 
June and October 1998 and December 1999. In June 
1999, high LAI values corresponded to high Batis 
maritima cover in addition to Monanthodoe cover. 

At Station II, LAI values were consistendy lower than 
the other stations (Table 6-6). However, increases in 
values were seen in August 1997, following the 
July 1997 event. The increased values occurred in the 
same area of the transect as the increased Batis maritima 
growth (Figures 6- 15b and 6-24c). High LAI values in 
June 1998 coincide with Monanthodoe littoralis and 
Borrichia frutescens cover (Figures 6- 12c and 6-24f). In 
June 1999, the greatest LAI values occurred where 
Salicomia bigelovii cover was high (Figures 6-1 2d and 
6-24J). Low LAI values correspond to high bare areas. 



LAI values at Station III were typically higher than the 
other stations (Table 6-6). The highest values were in 
the spring (Figure 6-25) and were lowest in areas with 
little to no vegetation. 



BlOMASS 
Batis Maritima 

Batis maritima biomass showed no obvious seasonal 
trends or event-mediated responses (Table 6-7). 
Biomass at all three stations ranged between 643 and 
3,878 g/m' Lowest values occurred at the Reference 
Station m May 1996 (1,016 g/nr) and February 1997 
(1,201 g/m"^. The lowest value recorded at Station II 
was also in May 1996 (643 g/m^. Low values occurred 
on the same dates at Station III as well (1,180 and 
805 g/m", respectively). Simultaneous occurrence of 
the low values between stations does not appear related 
to hydrographic conditions. At the Reference Station, 
biomass values peak in June 1997 (3,208 g/m"^, and 
gradually declined by more than half to 1,379 g/m' in 
June 1999. B. maritima biomass did not exhibit the 
same trend at the other two stations. 

Borrichia Frutescens 

Borrichia frutescens has the greatest range of biomass 
values and the greatest total biomass of the four 
species sampled (Table 6-7). Values between the 
stations were within the range of 1,948 to 10,412 g/m^. 
At all three stations, the highest values were seen at the 
beginning of the study period. At the Reference 
Station and Station II, high values of 8,053 and 
8,575 g/m*, respectively, were measured on May 1996. 
At Station III, the highest values measured were in 
February 1996 (10,412 g/m") and 1997 (9,848 g/m"). 
Biomass gradually decreased over the study period for 
all three stations. 



Chapter Six ♦ 6-37 



Table 6-6: Summary of LAI results from small scale GIS analyses. Values represent the percent of the transect 
covered by each LAI range. Only results from sampling dates prior or following composite hydrographic events causing 
measurable vegetation changes are included. May 96 data were included for comparison with other springtime values 



LAI 










Leaf Area Index Distribution (%) 










Range 


5/96 


6/97 


8/97 


11/97 


1/98 


6/98 


10/98 


12/98 


1/99 


6/99 


9/99 


12/99 


Reference Station 
























0-1 


47.1 


40.4 


39.8 


38.9 


2.0 


3.1 


40.3 


44.5 


29.6 


1.7 


87.0 


32.8 


1-2 


39.2 


34.1 


46.0 


49.6 


77.1 


78.4 


45.4 


47.7 


67.9 


80.0 


12.7 


55.6 


2-3 


11.8 


23.6 


13.1 


10.6 


32.6 


15.3 


14.3 


7.3 


2.5 


11.3 


0.3 


11.1 


3-4 


1.5 


1.8 


1.0 


0.7 


6.4 


2.7 





0.5 





4.1 





0.6 


4-5 


0.5 


0.1 


0.1 


0.1 





0.6 











2.9 








5-6 






































6-7 






































Station II 


























0-1 


46.0 


58.9 


55.2 


59.9 


52.9 


45.4 


50.5 


81.5 


72.0 


18.5 


91.3 


28.5 


1-2 


41.1 


37.0 


39.5 


38.2 


39.3 


46.0 


45.9 


17.2 


26.9 


75.5 


8.7 


61.9 


2-3 


11.6 


4.1 


5.3 


1.6 


7.6 


7.6 


3.7 


1.3 


0.9 


6.0 





8.5 


3-4 


1.4 








0.4 


1.0 


1.0 








0.2 








1.1 


4-5 






































5-6 






































6-7 






































Station III 


























0-1 


9.0 


16.0 


15.0 


20.1 


18.1 


16.9 


27.4 


26.6 


25.7 


21.6 


26.4 


26.9 


1-2 


19.7 


28.2 


21.7 


28.1 


29.3 


15.1 


37.4 


51.8 


30.9 


22.8 


42.0 


32.6 


2-3 


38.1 


36.8 


45.6 


37.7 


28.5 


45.6 


29.3 


18.3 


37.7 


34.1 


24.1 


25.9 


3-4 


25.2 


17.3 


16.7 


13.9 


18.8 


21.4 


5.6 


3.2 


5.3 


19.8 


7.0 


12.0 


4-5 


6.0 


1.7 


1.1 


0.2 


4.6 


0.7 


0.4 


0.1 


0.4 


1.6 


0.3 


2.5 


5-6 


1.4 











0.7 


0.2 














0.1 


0.1 


6-7 


0.5 




































6-38 



Vegetation Communities 



.^ 



(a) May 96 ^V 



# 



(b) June 97 




if^ 



(c) August 97 -'# 



(d) November 97 



^ 



I 10-1 
f— ]1-2 

■■2-3 
■|3-4 
^■4-5 
^■5-6 
^■B-7 
I I No Data 

\ 
(g) October 98 "p^ 



J\ 



^ 



(e) January 98 S^m 




c^ 



~lr 



^^ 



(h) December98 



O 

9 



^ 



(0 June 98 



r 



(i) January 99 



^ 

.r^ 



J 



(i) June 99 ^"^ 



Ir 



i>- 



(k) September 99 



(I) December 99 



e- 



JT 



Figure 6-23: Reference Station leaf area index (LAI) from GIS analyses. Results from sampling dates prior to and 
following the three composite hydrographic events that caused measurable changes in the vegetation are shown. May 
1996 is shown for comparison with other springtime sampling dates. Reference Station maps are shown for 
comparison, because this site was not affected by flow diverted by the demonstration project. 

Chapter Six ♦ 6-39 



(a) May 96 



^ 






"C 



<fi5 



(b) June 97 ^ 



uk 



(c) August 97 



\r 



(d) Novemb 


3r97 r 



(e) January 98 




^ 




|0- 1 

1 1 -2 






§■2-3 
m3-4 
■■4-5 
^■5-6 
^■6-7 
I I No Data 



(g) October 98 o 







x^ 



n. 



(h) December98 



^ 



(f) June 98 



.> 



x- 






(0 January 99 



O 

o 



^ 



<^" 



(i) June 99 




(k) September 99 



\ 



(I) December 99 



^ 



-^^nr!^ 



Figure 6-24: Station II leaf area index (LAI) from GIS analyses. Results from sampling dates prior to and following 
the three composite hydrographic events that caused measurable changes in the vegetation are shown. May 1996 is 
shown for comparison with other springtime sampling dates. Note increases in LAI in August 1997 following the July 
1997 event and increases in June 1999 following the October 1998 event. 



6-40 V Vegetation Communities 



Monanthocloe Littoralis 



<cwe Ltnoraits ^^ 

% ^ ^ 

(a) May 96 -^V^ (b) June 97 '' ^^ (c) August 97 

'^ ^ % 






(d) November97 



V % ^ 

^ ^Ty (e) January 98 '*'§*/ (f) June 98 ^^ 



^ 



I ^0- 1 -5-^ 

I 11-2 

^3-4 
^4-5 
■15-6 
^6-7 
I I No Data 



V ^" ■% 



^^%> 



"^^ ♦^^ 



-M* 



> ^ 



(g) October 98 ^IR> (h) December 98 i^ (i) January 99 a, 

V/ ^» "^^c 



(j) June 99 






'.^^ -ni^ ^1^ 

^■k (k) September 99 i^ (I) December 99 ^^ 

%, % % 

Figure 6-25: Station III leaf area index (U^l) from GIS analyses. Results from sampling dates prior to and following 
the three composite hydrographic events that caused measurable changes in the vegetation are shown. May 1996 is 
shown for comparison with other springtime sampling dates. Note increases in LAI in June 1999 following the 
October 1998 event. 

Chapter Six ♦ 6-41 







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6-42 



Vegetation Communities 



Monanthocloe Littoralis 

Monanthodoe littoralis biomass values were between 883 
and 3,706 g/m" for all three stations (Table 6-7). Low 
values were seen at all three stations in February 1997 
(< 1,772 g/m^. Biomass increased to near 3,000 g/m" 
in June 1997 at the Reference Station and Station II 
and declined to values between 1,885 and 2,213 g/m' 
in June 1999. At Station III, values increased by 
1,205 g/m^ from February 1997 to June 1998 and 
decreased again by 1,653 g/m* to the lowest value 
recorded (883 g/m^ in June 1999. 

Distichlis Spicata 

Distichlis spicata exhibited the lowest biomass range of 
all four species sampled (314 to 3,557 g/m^ 
(Table 6-7). Values ranged between 1,047 and 
2,199 g/m" at the Reference Station. At Station II, 
values were similar to the Reference Station with the 
exception of one high value recorded in January 1998 
(3,557 g/m'^. Values at Station III were considerably 
lower than the other two stations (314 to 666 g/m"^, 
and on three sampling dates, coverage of D. spicata was 
so sparse that monospecific stands could not be found 
for sampling. 



ROOT: Shoot Ratios 

Batis Maritima 

RS ratios of Batis maritima were lowest in May 1996 at 
all three stations (0.16 to 0.22) (Table 6-8). The ratios 
gradually increased until January 1998 at the Reference 
Station (0.83) and Station III (1.72) and continued to 
increase until June 1998 at Station II (1-27). Values 
decreased gradually at the Reference Station and 
Station II to values of 0.36 and 0.54, respectively. The 
ratio peaked again in January 1999 at Station III but 
then declined to a final recorded value of 0.61. 
Twenty-two of the twenty-four (98%) ratios measured 
were between 0.1 and 1.0, indicating that for most of 
the demonstration period, the plant biomass above the 



ground was proportionately larger that below the 
ground. 

Borrichia Frutescens 

At the Reference Station, R:S ratios were greatest in the 
summer months and lowest in the winter (Table 6-8). 
This station also exhibited the greatest range in ratios 
(0.2 to 0.9). At Station II, values peaked in June 1997 
(0.83) and then gradually decreased by 64% to 0.30 at 
the end of the study period. Values at Station III 
showed litde variation, with the range in values being 
limited to 0.2 to 0.4. The lowest values measured at all 
three stations were in February 1997. All values 
recorded were between 0.2 and 0.9. 

Monanthocloe Littoralis 

No apparent seasonal trend in the R;S ratios of 
Monanthocloe littoralis could be seen at the three stations, 
and ratios between stations were variable (Table 6-8). 
At the Reference Station, values remain relatively 
constant between 0.1 and 0.6. A gradual decrease in 
ratios occurred from June 1998 to June 1999. A peak 
ratio was measured in June 1997 at Station II (1.3), and 
in June 1998 at Station III (1.1). The peaks did not 
correspond to changes in freshwater input. Values for 
all three stations ranged between 0.1 and 1.3. 

Distichlis Spicata 

R:S ratios at the Reference Station and Station II 
exhibited similar patterns for Distichlis spicata, although 
the range of values was greatest at Station II 
(Table 6-8). Peak ratios were recorded in June 1997 
and June 1999, foUowing periods of hea\'7 rainfall. 
Lowest values were measured in June 1998 during a 
period of Litde to no rainfall. 



Chapter Six ♦ 6-43 



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6-44 



Vegetation Communities 



DISCUSSION 



Inorganic Nitrogen 



Salinity 

In general, open water salinity values appeared to be 
influenced by precipitation and flow-mediated 
freshwater inputs. Typically, salinity values were 
decreased by freshwater inputs and increased by 
drought periods. The greatest peaks in salinity 
occurred during dry periods. The extent and duration 
of the freshwater event determined the degree to which 
the salinity fell, and the time span over which the 
salinity remained low. For example, a summer event in 
1997 reduced open water salinity levels by 40 ppt at the 
Reference Station. However, the salinity had increased 
by 30 ppt only two months following the event. The 
magnitude of the increase was most likely due to the 
timing of the event. The event occurred during the 
summer, and, after the precipitation ceased, air 
temperatures were high and resulted in rapid 
evaporation. In contrast, the October 1998 event 
reduced open water salinity values at the Reference 
Station from 33 ppt in June 1998 to 11 ppt ia 
December 1998. Salinity levels remained low 
throughout the winter and spring 1999, most likely 
because the event occurred during the cooler part of 
the year, and because there was steady rainfall during 
the beginning of the year. 

Pore water salinity concentrations were almost always 
higher than open water values. The periods when 
values were similar were because the transects or part 
of the transects were flooded with open water. This 
trend was similar to that described by Hackney and 
De la Cruz (1978) for a Mississippi tidal marsh. The 
increased pore water values appear to be due 
evaporation of water from the soil, as well as lack of 
dilution from fresh water, as the highest pore water 
values were seen during drought periods (e.^., 85 to 
92 ppt in December 1999, which had 0.25 cm of rain). 
Changes in pore water salinity concentrations were 
usually reflected by changes in open water salimty 
values. 



Open water NH4'^ and NO^" + NO,' values showed 
only small responses to freshwater inundation. A 
problem with evaluating the impact of the project on 
these values was that it must be assumed that 
instantaneous nitrogen levels measured on different 
sampling dates were representative of the same body of 
water. However, in reality, the channel water was not 
stagnant, and moved continually based on tidal 
oscillations. 

These problems were probably minor in the analysis of 
vegetation responses as the nitrogen available for plant 
uptake was found in the soils, which, unlike channel 
water, did not move. The role of pore water nitrogen 
in determining the biomass and productivity of salt 
marsh vegetation has been investigated repeatedly. In 
general, it has been demonstrated that nitrogen 
fertilization of marsh plants has significandy increased 
plant biomass, indicating nitrogen as a limiting nutrient 
in estuaries. Several investigators have concluded that 
nitrogen scarcity is responsible for stunting the growth 
oi Spartina altemiflora (Broome et al. 1975; Gallagher 
1975; VaHela and Teal 1974; VaUela et al. 1978). 
Mendelssohn (1979) demonstrated that nitrogen 
fertilization increased the aerial living standing crop of 
the short form oi S. altemiflora from 49% to 172%. 
De La Cruz et al. (1979) found significant increases in 
the annual aboveground net primary productivity of 
four tidal marsh communities seven months after 
ammonium nitrate fertilization. 

In this study, pore water NH4* levels increased 
dramatically from the beginning to the end of the study 
period. Most increases were seen after the October 
1998 composite hydrographic event. The change does 
not appear to be flow related, as similar increases were 
measured at all three stations. However, the 
concentrations were most likely too high to be purely 
rainfall meditated. The change may have been a result 
of cyanobacterial mats that formed on the soil surface 
following the event. Many cyanobacteria species are 
capable of taking atmospheric nitrogen and fixing it 
into a biologically available form NH4*. These mats 
remained on the soil surface for over a year (they were 
at the stations through the December 1 999 sampling 



ChapUrSix ♦ 6-45 



date but were dried up by June 2000). In general, 1999 
was a relatively wet year, and the moisture might have 
attributed to the survival of the mats. The persistence 
of the mats for an extended time could explain the 
unusually high soil NH^^ levels. Unfortunately, the 
high levels were not seen until the end of the study 
period, and any subsequent positive effects on the 
vegetation would hav^e taken several months to 
mamfest. 



Vegetation 

The most significant vegetation changes seen as a 
result of hydrographic events through the channels 
were the germination and establishment of the annual 
succulent Salicomia bigelovii. The relative success of this 
plant appears to provide relevant information 
regarding the timing and quantity of fresh water 
needed to promote sexual (seed) coloni2ation in 
hypersahne salt marshes. Because the species is an 
armual and reproduces primarily by seeds, successful 
establishment can only occur if soil salinity 
concentrations are reduced to a level that alleviates the 
osmotically induced seed dormancy. 

In salt marsh habitats, the ability of halophytes to 
reproduce sexually (seeds) is critical for the success of 
plant populations (Ungar 1991 and Allison 1996). 
While vegetative growth can expand plant cover in a 
particular area, seeds are necessary for establishment at 
locations distant from neighbors. Seeds also allow for 
reestabUshment following disturbance events like 
flooding, drought or burial that oftentimes result in 
adult plant mortality. Quick recovery is only possible 
through seeds, which can withstand the disturbance by 
remaining dormant and then germinating afterwards. 

In this study, the timing of soil sahnity reduction was 
found to be critical. Hydrographic events which 
lowered soil salinity values during the late fall/early 
winter allowed the successful germination and 
establishment oi Salicomia bigelovii during the spring and 
summer, independent of whether or not the 
intermediate seasons were wet or dry. The degree of 
survival was most likely a fiinction of air temperatures 
and evaporation rates, which were relatively low in the 



winter compared to summer and did not increase as 
rapidly. VCTien summer approached and soil salinity 
increased, the seedlings were large enough to withstand 
the increasing salinity concentrations. If an event 
occurred in the late spring or summer, the seeds might 
have germinated, but unless the soU salinity was kept 
suppressed by sequential diluting events, the seedlings 
would not sunave and establishment likely failed. 

Flooding events through the Rincon Overflow 
Channel occurring in the late fall significandy increased 
the number of established plants seen the following 
spring compared to the number seen after only a 
precipitation event. In June 1999, Salicomia bigelovii 
cover at Station II (52%) was 26% greater than that 
seen at the Reference Station (26%), while cover in 
June 1998 was approximately the same at the two 
stations (11 to 13%). Heavy rains occurred during fall 

1997 with no flow event, hence the approximately 
equal cover at the two stations. While m fall 1998, 
heavy rains and a flow event occurred through the 
Rincon Overflow Channel, possibly leading to a 
doubling in cover at Station II compared to the 
Reference Station. The flooding of Station II was 
reflected in the soil salinity levels, which were 6 to 
20 ppt. These values are 1 to 11 ppt lower than the 
Reference Station and Station III, respectively, 
immediately following the flow event. 

The lack of a significant difference between Salicomia 
bigelovii covet at Station III between June 1998 and 
June 1999 indicated that diverted fresh water in the 
channels might not have affected more than just the 
lower (adjacent) portions of the transect at Station III. 
Most likely, during a major event such as the October 

1998 event, most flow diverted through the Nueces 
Overflow Channel backed up at the intersection of the 
upper and central segments of Rincon Bayou by the 
private road crossing, and a majority of this volume 
likely passed through the Rincon Overflow Channel. 
Some amount of fresh water reached Station III, as 
open water salinity was 8 to 1 1 ppt lower than the 
other two stations following the October 1998 event, 
but this water likely remained channelized. However, 
open water salinity values were 8 to 9 ppt and 5 to 

6 ppt higher following the July 1997 and September 

1999 events, respectively. The higher salinity values. 



6-46 



Vegetation Communities 



even after major hydrographic events, may be 
indicative of increased tidal influence, as this station 
was closer to Nueces Bay. The smaller responses seen 
in S. bigelovii cover at the Reference Station and Station 
III compared to the response seen at Station II after 
the 1998 event appear to be precipitation-mediated and 
limited by a lack of freshwater inundation. 

Small scale GIS analyses indicated that the 
establishment of Salicomia bigelovii corresponded to a 
decrease in bare area at the Reference Station and 
Station II. At Station II, decreases in bare area were 
proportional to increases in cover. LAI maps 
produced from the small scale analyses further 
supported the increase in vegetation at both the 
Reference Station and Station II during spring 1998 
and 1999. Weilhoefer (1998) and Dunton et al. (2000) 
have previously dociimented establishment and 
expansion of vegetation cover and subsequent decrease 
in bare area after freshwater inundation in the lower 
Nueces Delta. Decreases in bare area could have direct 
affects on the functionality of the marsh. 

Other studies have noted that halophyte species, which 
can occupy bare areas following disturbance events 
through sexual (seed) reproduction, can act to reduce 
soil salinity concentrations by shading the soil surface 
and by actively uptaking salts. The reduction in soil 
salinity eventually allows the establishment of 
dominant and persistent perennial species (Bertness 
etal. 1992). The establishment of these species is 
necessary as they pro\ade stable habitat for many for 
many small organisms, including crabs, molluscs and 
small terrestrial animals such as rats. The marsh 
vegetation also serves as habitat and food for a variety 
of permanent and migratory birds (Henley and 
Rauschbauer 1981). Successful long-term 
establishment provides large amounts of plant biomass 
to the detrital food-web, which can then be utilized by 
microorganisms and other small animals such as snails 
(Marples 1966). This is an especially important role of 
annual plants because all of their aimual biomass 
production eventually becomes detritus. Marsh 
vegetation contribution to the detrital food-web is 
essential as it serves as a critical link between primary 
and secondary trophic levels (Burkholder and 
Burkholder 1956; Odum and Wilson 1962; Teal 1962). 



Vegetative occupation of bare space is critical as the 
vegetation acts to stabilize marsh sediments, thereby 
sheltering the metropolitan area of Corpus Christi 
from extensive flood damage and erosion. Without 
vegetation expansion into bare space, the functionality 
of the marsh would be compromised. 

hatis maritima cover at the Reference Station and 
Station III changed seasonally, with peak cover in the 
summer and declines in the winter. This seasonal 
trend appears to be due to natural variation and 
corresponds to observations made by Weilhoefer 
(1998) in the lower Nueces Delta. B. maritima is a 
highly salt tolerant perennial succulent, which may 
explain its high cover during the summer. The plant 
expanded in cover during the spring and early summer 
when soil salinity levels were usually low and was then 
able to maintain a high cover during the summer 
months, although active growth may not have occurred 
during the hot, dry season. The greatest percent cover 
observed at the three stations, however, was during the 
summer of 1997, following a spring with over 52.1 cm 
(20.7 inches) of precipitation. The rain was steady over 
a five-month period and was not due to a major 
hydrographic event. No other spring during the study 
period experienced a similar rainfall pattern. Although 
B. maritima is capable of surviving at elevated salinity 
concentrations, increases in cover occur when gradual 
freshwater inputs alleviate soil salinity levels. 

After a major composite hydrographic event in 
July 1997, percent cover of hatis maritima temporarily 
increased in August but then declined at all three 
stations. After the summer 1997 event, the following 
fall, winter and spring were relatively dry, potentially 
keeping cover low at all three stations during June 
1998. Cover increased again in June 1999 at die 
Reference Station and Station III; cover remained low 
for over two years at Station II. The continual 
suppression of B. maritima growth at Station II might 
reflect this species intolerance to waterlogging and 
anaerobic soils. Freshwater-mediated decreases have 
been noted in a species with similar morpholog}', 
Salicomia virginica (Zedler and Beare 1986; Weilhoefer 
1998). Continual flooding occurred during the 
fall 1998 and diroughout 1999. At Station II, the 
vegetation began at the mean water line, so, after 



Chapter Six ♦ 6-47 



flooding, fresh water drained slowly. This effect was 
magnified at Station II after an event activating the 
Rincon Overflow Channel, as a larger amount of water 
affected this station as compared to the other stations. 
It is important to note that B. maritima cover began to 
increase following the composite hydrographic event 
during the summer of 1999, as the winter months were 
dry and the waterlogged conditions potentially 
alleviated. At Station III, the vegetation zone began 
above the mean water line and so was not flooded as 
easily by channel water. Furthermore, water from 
precipitation did not produce standing puddles that 
could waterlog the soils. 

The decrease in cover oi Monanthocloe littoralis at the 
Reference Station and Station II following the summer 
1999 composite hydrographic event was most likely 
due to flooding of the transects. During the time of 
the floods, the transect soils were covered by thick, 
green cyanobacterial mats. When the waters flooded 
the transects, the mats were lifted from the soils; when 
the waters retreated, the mats were left lying on top of 
most of the vegetation. The thick mats most likely 
caused decreases in plant cover by passively shading 
the vegetation, making interception of light necessary 
for photosynthesis almost impossible. Fortunately, 
cover of both Batis maritima and M. littoralis began to 
quickly recover after the event. In this instance, 
although flooding initially served as a disturbance 
resulting in decreases in adult plant cover, the 
UkeHhood of long-term successful establishment might 
not have been affected by the event. However, as 
noted by AUison (1996), the recovery may depend 
upon the presence of occasional freshwater inxmdation. 

Biomass values and R:S ratios did not show obvious 
freshwater mediated responses. Previous studies have 



suggested that plants living in hypersaline soils invest 
more energy into below-groimd tissues in order to 
cover a larger area of soil to obtain water (BrugnoU and 
Bjorkman 1992; Kuhn and Zedler 1997). However, 
the physiological capacities of the plants that control 
their salinity tolerance can vary with each species, and 
some species may allocate different amounts of 
biomass to the below-ground tissues depending on 
their individual ability. Additionally, waterlogging of 
soils may cause below-ground material to die in some 
plants and not in others, thereby decreasing the 
R:S ratio. The analyses were further complicated as a 
need for increased nutrient uptake might also have 
resulted in the plants increasing their below- ground 
tissues, especially when increases in nutrients occurred 
after precipitation or flooding events. 



SUMMARY 

The increase of fresh water into the channels and tidal 
flats of the upper Nueces Delta positively impacted the 
emergent vegetation by decreasing salinity, allowing for 
annual seed reproduction, decreasing bare area and 
increasing vegetative expansion of plant cover. 
Although similar responses were seen at the Reference 
Station following heavy precipitation, the effects were 
accentuated at Station II, which was direcdy impacted 
by flow through the Rincon Overflow Channel. The 
project demonstrated the sensitivity of vegetation to 
salinity levels and indicated that periodic freshwater 
inundation could alleviate the stressful condition 
imposed by hypersalinity. Without fresh water 
flooding, soil salinity concentrations at several 
locations in the study area would have likely increased 
to toxic levels and resulted in plant mortality. 



6-48 



Vegetation Communities 



CHAPTER SEVEN 

Synthesis and 
Conclusions 



"Then what of this river that having arisen 
Must find where to pour itself into and 
empty?" 

♦♦♦ Robert Frost, Too Anxious for Rivers 



The Nueces Delta is one of the most extensive marshes 
on the Texas Gulf Coast. It is an integral component 
of the Nueces Estuary, providing economically and 
ecologically valuable habitat and food for many 
estuarine and marine plant and animal species. In 
1997, the worldwide average economic value of 
17 ecological services provide by an estuary was 
$9,000 per acre per year (Costanza e( a/. 1997). In 
Texas, the total of only two recognized estuary 
functions (commercial and recreational value of 
fisheries) was about $5 billion dollars for the one 
million acres of estuarine area in the State (Robinson 
et al. 1995), or about $5,000 per acre per year. 

The flora and fauna of the Nueces Delta depend upon 
periodic freshwater inundation events to maintain their 
ecological functions. However, over the past century, 
increases in the human population in the Coastal Bend 
region of Texas has intensified the demand for fresh 
water to meet agricultural, municipal and industrial 
needs. From the combined effects of reservoir 
construction, changes in land use patterns, increased 
ground water withdrawals and other human activities, 
the average annual volume of fresh water diverted into 
the upper Nueces Delta since 1982 has been reduced 
by over 99% from that before 1958 (Irlbeck and Ward 
2000). Over time, this decrease in freshwater inflow 
has created a non-functioning estuarine ecosystem in 
the Nueces Delta. The natural freshwater deficit 
imposed by evaporation has been magnified by 
decreased riverine inflow, resulting in hypersaline 
channel waters and soils. As a result, a "reverse 
estuary" condition has developed where the lowest 
salinitjr values are near Nueces Bay and the highest are 
in the upper delta. While many estuarine species 
tolerate this hypersaUne environment, prolonged 



Chapter Seeen ^ 7-1 



periods of salinity stress have limited active growth and 
reproduction in the Nueces Delta, leading to lower 
biological productivity and less species diversity. 

This demonstration project was designed to increase 
the opportunity for freshwater flow events &om the 
Nueces River into the upper Nueces Delta assuming 
that resultant hydrographic changes would beneficially 
affect the ecology of the upper delta. Reclamation 
initiated the demonstration project in 1993 with the 
excavation of two overflow channels and then 
monitored the changes in hydrography, the water 
column, benthic communities and vegetation 
communities. 



CHANGES IN HYDROGRAPHY 

During the 50-month period when the overflow 
channels were open, over 8,810 10' m' (7,142 acre-ft) 
of water was diverted from the Nueces River into 
Rincon Bayou. Of this total amount, only about 
1,204 10' m' (976 acre-ft) wovdd have been diverted 
without the demonstration project features. Therefore, 
the total volume of freshwater inflow to the upper 
Nueces Delta was increased by about 732% from what 
would have occxirred without the project. 

In addition to increased inflow, the demonstration 
project features also increased the distribution of firesh 
water within the tidal flats of the upper delta. On five 
different occasions during the demonstration period, 
the tidal flats of the upper marsh were supplied with 
diverted fresh water. Without the project features, no 
natural inflow event would have direcdy freshened 
these areas. 

These changes occurred because the demonstration 
project features lowered the minimum flooding 
threshold for the Nueces River firom 1.64 m (5.4 ft msl) 
to about m mean sea level. This change not only 
allowed more frequent river diversions into Rincon 
Bayou and the upper delta, but also provided the 
opportunity for other non-riverine elements Uke wind 
and tide to force frequent exchanges between the upper 
delta and the Nueces River. As a result, near continual 
(daily) exchange between the river and delta was 



observed during the demonstration period. Before the 
project, such interactions were limited to only 
extremely infrequent river inflow events. 



EFFECTS ON SALINITY 

The demonstration project gready lowered short-term 
salinity concentrations in the upper and central 
segments of Rincon Bayou, as indicated by results of 
the hydrographic analysis (Chapter 3) and subsequent 
biological response analyses (Chapters 4 through 6). 
The demonstration project also affected the overall {i.e., 
long-term) salinity gradient of Rincon Bayou. 
Unpublished long-term saUnity data from selected 
stations in the Nueces Delta and Nueces Bay were 
obtained from Dr. Terry VCTiidedge (University of 
Alaska Fairbanks) and Dr. Paul Montagna (University 
of Texas Marine Science Institute) (Figure 7-1).). 
These data covered the period of January 1992 through 
December 1999 and were made available from projects 
sponsored by the Texas Water Development Board, 
City of Corpus Christi, Corpus Christi Bay National 
Estuary Program and Texas Sea Grant Program. This 
8-year interval was divided into two nearly equal 
periods before and after the Nueces Overflow Channel 
was completed (October 26, 1995). The before-project 
period was about 3.8 years long, and the after-project 
period was about 4.2 years long. 

The average salinity at each selected station was 
calculated for each month. Monthly averages at each 
station were then averaged for each period before and 
after the demonstration project to smooth the year-to- 
year variation and compare means. These values were 
then plotted along their respective channel segment 
lengths, beginning at where the Interstate Highway 37 
(IH 37) bridge crosses the Nueces River to a point in 
Nueces Bay just west of White Point (Figure 7-2). 

In the upper and central segments of Rincon Bayou, 
the average salinity gradient changed dramatically after 
the opening of the Nueces Overflow Channel. During 
the period before the demonstration project, there was 
no regular interaction with the Nueces River and 
salinity concentrations in the delta were highest in 
upper Rincon Bayou (Figure 7-2a). This condition may 



7-2 ^ Synthesis and Conclusions 



DEMONSTRATION PROJECT 
STUDY AREA 



Approximate Scale 
Kilometers 




^^"■^^ 



Figure 7-1 : Selected stations used for analysis of long-tetTn salinity changes in Rincon Bayou. 



be termed a "reverse estuary," because the gradient was 
opposite what would be expected from a natural 
(undisturbed) system in which salinity concentrations 
would be lowest in the upper delta {e.g., at Station 66). 
For the period after the demonstration project, the 
salinity gradient reverted to a more natural pattern for 
much of it's reach, particularly within the upper and 
middle Rincon Bayou segments (or the first 5 km from 
the IH 37 bridge) (Figure 7-2b). This change was due 
to increased diversions of fresh water through the 
demonstration project features. Without the 
demonstration project, average salinity concentrations 
in upper Rincon channel during the second period 
would have remained strongly hypersaline (likely 
greater than 50 parts per thousand (ppt) instead of the 
observed range of 21 to 28 ppt. 

As expected, the demonstration project had no obvious 
effect on the salinity pattern of the Nueces River, nor 
was it designed to (the project was found to only divert 
approximately 2% of total river flow during a given 



hydrographic event (Ward 2000)). During both 
periods, salinity concentrations in the river channel 
increased with distance downstream as riverine fresh 
water (e.g., at Station 68) mixed with salt water from 
Nueces Bay (e.g., at Station 7) (Figure 7-2). 

From comparison of the actual average salinity values 
in the river and bayou between both periods (i.e.. 
Figures 7-2a and 7-2b), it is apparent that salinity 
concentrations were generally higher during the after- 
project period than before the project. For example, in 
the Nueces River, this difference was about 3 parts- 
per-thousand (ppt) in the middle of the river 
(Station 4b) and about 8 ppt in the open bay 
(Station 7). This observation does not indicate that the 
demonstration project increased average salinity 
concentrations in the river but more likely is 
attributable to a greater freshwater influences on the 
river from both river flow and precipitation during the 
period before the project than after (Figure 7-3). 



Chapter Seven ^ 7-3 



(a) Before the demonstration project (1992 through 1995) 



60 



50 



Q. 
Q. 



40 



CD 
CO 

Q) 
O) 
TO 

i_ 
(U 
> 
< 



30 



20 



10 





RR 
















Rincon Bayou 

Nueces River 




1 




. ' 


























% 


5.^ 


RR 

' r 






















^S4- 


^ \ 


\ 






1 Nueces Bay 






IH'^7 










"^1 




cr\ -1 




^ 
^ 




Brie 


Ige 
RR 


RR 








V 

6 


8 










48--'" 


1 


.^AT- 




1 Nueces Bay 

1 1 1 



6 8 10 12 14 

Distance in channel (km) 



16 



18 



20 



22 



(b) After the demonstration project (1996 through 1999) 



60 



50 



S 40 
>. 
g 
TO 30 

0) 
O) 
TO 

\ 20 



10 - 























Rincon Bayou 

Nueces River 




1 












RR 

\\ 






















.^ 


/\ 


\ 








Nuec 


as Bay 










7^ 


V 




\ 


Nil 




1 


IS ■: 




a.4^ 


^ 


/ 


IH3" 
Bridg 


RR / 

;7 










RR 

J 


AN'" 


^, — ' 


A 


ueces Ba 


y 


6 


1 









48—^ 


» ^ 








1 





6 8 10 12 14 

Distance in channel (km) 



16 18 20 



22 



Figure 7-2: Long-term average salinity values for selected stations in the Nueces River and Rincon Bayou before 
(a) and after (b) implementation of the demonstration project. Station numbers coincide with those in Figure 7-1 . 
Salinity values for the upper-most reach of Rincon Bayou (near Station 67) during the before-project period (a) were 
unavailable but were estimated by extrapolation to be approximately 45 ppt. RR indicates the point at which each channel 
is crossed by a railroad. 



7-4 V Synthesis and Conclusions 



(a) Total annual flow 
in the Nueces River 
(at Calallen gauge) 

600000 




(b) Total annual precipitation 
(at Corpus Christi International 
Airport) 

120 



E 

*«— 

c 

CD 

o 



100 
80 i 
60 
40 
20 
J-ls, 



(c) Total annual flow 
into Rincon Bayou 
(at Rincon gauge) 

7000 



i^ 6000 

^ 5000 
o 

-™. 4000 - 

O 3000 

TO 2000 

° 1000 




^E^ 



XI 



a 



1992 1994 1996 1998 



1992 1994 1996 1998 



T T 

1992 1994 1996 1998 



Bef ore-project annual mean 
After-project annual mean 



Figure 7-3: Total annual cumulations of selected freshwater sources affecting the Nueces Delta during the period 
1992 through 1999. Flow in Rincon Bayou for the before-project period was estimated using dally stage data from the 
Calallen gauge and the methodology described by Iribeck and Ward (2000). Total flow into Rincon Bayou for the after- 
project interval of October 26, 1995, through May 15, 1996, was unavailable, but was estimated to be approximately 
123 10^ m^ (100 acre-ft). The average annual flow In the Nueces River for the first period was 224,581 10' m' 
(182,053 acre-ft) but only 127,678 10' m' (103,500 acre-ft) for the second. The average annual precipitation for the before- 
project period was 94.2 cm (37.1 in) but only 74.7 cm (29.4 In) for the after-project period. Finally, the annual mean 
freshwater flow into Rincon Bayou for the first period (1 ,931 10' m', or 1 ,565 acre-ft) was nearly equal to that for the period 
after (1 ,854 10' m', or 1 ,503 acre-ft). 

Note; 1 acre-fl = 1.2336 10' m' 



In summary, the effects of the demonstration project 
on the long-term salinity gradient in Rincon Bayou 
were measurably significant. In a relatively short period 
of time (only 4.2 years after the opening of the Nueces 
Overflow Channel), the "reverse estuary" salinity 
gradient in the upper delta before the demonstration 
project reverted to a more natural form, with average 
salinity concentrations in upper Rincon Bayou 
becoming the lowest in Nueces Delta. 



BIOLOGICAL RESPONSES 

The influence of fresh water on the salinity 
concentrations of the water and soils of the upper 
Nueces Delta appeared to be the most important 
parameter affecting the biological response of estuary 
organisms in the delta. 



Water Column Productivity 

The inflow of river water into the upper Nueces Delta 
imported vital nutrients (nitrogen, phosphorus and 
silicon) required for plant growth into the channels and 
ponds of the marsh ecosystem. During the 
demonstration period, phytoplankton in the water 
column and on the surface of the sediments rapidly 
responded to the inputs of riverine nutrients with 
increased growth rates and accumulation of biomass. 
In addition, the freshwater inflow also reduced salinity 
concentrations and lowered the osmotic stress on these 
organisms. The increased primary production rates 
were especially prominent during periods when salinity 
was less than 50 ppt. The assimilation index {i.e., the 
relative amount of growth per cell) was also generally 
higher during periods of low salinity, indicating that 
inherent growth rates were also increased by project 
diversions. 

The species composition of phytoplankton apparently 
remained dominated by primarily small diatoms during 



Chapter Seven ^ 7-5 



most of the monitoong period. However, several 
observations of blooms of other phytoplankton were 
noted immediately after freshwater inflow events. 
These blooms were typically comprised of single celled 
blue-green algae (not of the filamentous cyanobacteria 
of algal mats) normally present in fresh water or very 
low saUnity environments. These blooms were also 
short-lived, persisting no more than a few days. The 
presence of these blooms did not occur in the study 
area prior to the demonstration project except xmder 
natural freshening events that occurred every several 
years. The more firequent presence of these blooms in 
the upper and central Rincon Bayou were an indication 
that the water column ecosystem was showing a more 
typical response to freshwater inflow. 

An additional indication of improved conditions for 
phytoplankton growth during and after freshwater 
inflow events was shown by an increase in the 
N;P ratio (dissolved inorganic nitrogen to phosphate 
ratio), which would provide a relative increase of 
nitrogen needed for plant growth. These increases 
following inflow events indicate that relatively smaller 
amounts of denitrification probably occurred during 
and following the inflow events. 



Benthic Communities 

Benthos consists of two main types of infauna, which 
have different ecological roles in marine ecosystems 
and respond to inflow at different spatial and temporal 
scales. These are the larger macrofauna (organisms 
greater than 0.5 mm in length) and smaller meio fauna 
(between 0.063 and 0.05 mm in length). Macrofauna 
have planktonic larval dispersal and indicate effects of 
the demonstration project over larger spatial scales and 
longer temporal scales. Meiofauna have direct benthic 
development and generation times as short as one 
month, thus indicate effects over smaller spatial scales 
and shorter temporal scales. 

Diverted fresh water lowered salinity concentrations 
which triggered bursts of benthic iavertebrate 
productivity as indicated by increases in density and 
biomass. Salinity levels between 20 and 45 ppt 
appeared to correspond with high macrofauna biomass 



(> 2 g/m^. Macrofaunal abundance also increased 
with increasing biomass. Meiofaunal biomass and 
abundance increased when salinity values were between 
10 and 40 ppt, with the greatest numbers being seen in 
the salinity range of 1 8 to 22 ppt. Biodiversity 
increased several months (3 to 6) following inflow 
events, indicating more species were able to utilize the 
marsh habitat. The macrofauna and meiofauna 
responded to inflow with similar patterns, indicating 
both trophic levels of benthos were responding 
positively to inflow events. Most importantiy, the 
lowest biomass and abundance values occurred during 
times when there were no flow events. Additionally, 
strong seasonal increases of biomass occurred during 
the spring, when salinity concentrations were lowest. 
In contrast, during summer when salinity values were 
highest, density and biomass were always lowest. 



Vegetation Communities 

The most significant changes in the emergent 
vegetation also coincided with changing salinity regimes 
caused by flow through the demonstration features or 
by direct precipitation. During sampling periods with 
no hydrographic events, soil salinity levels were as high 
as 80 to 90 ppt, and water column salinity was upwards 
of 40 to 60 ppt. Following a large hydrographic event, 
surface and soil salinity concentrations were reduced, 
with the degree and duration of salinity suppression 
being a function of the timing and duration of the 
hydrographic event. Major hydrographic events 
lowered open water salinity values by over 40 ppt at 
some stations, and smaller, precipitation-mediated 
events exhibited smaller decreases. 

Hydrographic events that alleviated soil salinity levels 
during the late faU/early winter allowed the successfiil 
germination and establishment of Salicomia higelovii 
during the spring and summer, independent of whether 
or not the seasons were wet or dry in regards to 
precipitation. Because this plant species is an annual 
succulent that reproduces primarily by seeds, successfiil 
establishment seemed to occur only if soil salinity 
concentrations were reduced at the appropriate time of 
year to a level that covdd alleviate osmoticaUy induced 
seed dormancy. 



7-6 V Synthesis and Conclusions 



Most importantly, the establishment of Salicomia higelovii 
corresponded to a decrease in bare area at all three 
vegetation sampling stations, with the decreases in bare 
area being proportional to the increase in S. higelovii 
cover. Decreases in bare area coxild have direct affects 
on the functionality of the marsh. Other studies have 
noted that halophyte species, which can occupy bare 
areas following disturbance events through sexual 
(seed) reproduction, can act to reduce soil salinity 
concentrations by shading the soil surface and by 
actively up-taking salts. The reduction in soil salinity 
can eventually allow the establishment of dominant and 
persistent perennial species. These plants can then 
provide stable habitat and food for many small 
organisms and for a variety of permanent and 
migratory birds, as well as provide large amounts of 
plant biomass to the detrital food-web, which boosts 
productivity of higher trophic levels. Without the 
expansion of vegetation into bare space, the 
functionality of that marsh area would be 
compromised. 

The response of Salicomia higelovii also differed after 
freshwater inflow events when compared to those that 
were mainly precipitation-mediated. In the late fall 
1998, a major flow event flooded the Rincon Overflow 
Channel and significandy increased the number of 
plants that emerged the following spring compared to 
the number seen after a precipitation only event. In 
June 1999, total Salicomia higelovii cover at Station II was 
52%, which was 26% greater than that seen at the 
Reference Station. In June 1998, prior to the October 
1998 composite hydrographic event {i.e.. Events 21 
through 27), percent cover at the two stations was 
approximately equal (11% and 13%). The similar 
amount of cover during the spring 1998 could be 
explained by heavy rains (> 23 cm, or 9 inches) which 
occurred during fall of 1997, presumably affecting both 
stations the same amount, although Station II was also 
affected by fresh water diverted through the Rincon 
Overflow Channel. 

Peaks in the percent cover of the perennial succulent 
^atis maritima occurred after gradual precipitation 
inputs, but decreased after major hydrographic events 
which flooded the station. The decline was most likely 
due to the species' inability to tolerate waterlogged and 



anaerobic soils for extended periods. Increases in cover 
were noted several months following a major event if 
the soils had time to dry {i.e., no other flooding events 
occurred). 

In summary, freshwater inputs via precipitation or 
project diversions reduced salinity concentrations in the 
upper Nueces Delta, and vegetation cover increased as 
a result. Although large flooding events initially served 
as a disturbance resulting in decreased adult plant 
cover, it appears that the likelihood of long-term 
successful establishment might be enhanced by the 
increased opportunity of freshwater inflow. 



INTEGRATION OF PROJECT 
EFFECTS 

The overall effects of the demonstration project on the 
ecology of Rincon Bayou and the upper Nueces Delta 
were positive to the environment (Table 7-1), and were 
attributable to the re-introduction of fresh water. Prior 
to the demonstration project, Rincon Bayou was not 
direcdy connected with the river {i.e., it was a dead-end 
channel). Average salinity concentrations were 
consistentiy higher in the upper delta than in Nueces 
Bay. These conditions were alleviated in the upper and 
central segments of Rincon Bayou by increased 
freshwater flow through the Nueces Overflow 
Channel. 

The increased opportunity for freshwater inflow also 
changed nutrient cycling and primary productivity in 
the upper delta. Generally, nitrogen occurs in the water 
column in three main forms: ammonia, nitrate and 
nitrite. Without inflow, the dominant form of nitrogen 
in the upper delta was ammonia, which was likely 
derived from the recycling of (decaying) organic matter. 
Also, water levels were generally low, as shallow as only 
a few centimeters, and the substrate was covered by 
mats of filamentous blue-green algae {i.e., 
cyanobacteria). Although productivity (per unit 
volume) from these mats was high, the total volume of 
production was low because the total amount of water 
was small. Furthermore, cyanobactena are not food 



Chapter Seven ^ 1-1 



Table 7-1 : Summary of the effects of the demonstration project on the upper Nueces Delta. 



ATTRIBUTE 



Before 



After 



Geomorphology 
Salinity gradient 
Nutrient cycling 



Dead-end Flow-through with free exchange 

Higher in the upper delta than in the bay Lower in the upper delta than in the bay 
Recycled nitrogen New and recycled nitrogen 



Primary production Low In marsh 

Secondary production Constrained by dry conditions 

Habitat utilization Constrained by dry conditions 



Higher in marsh 
Increased by flow events 
Increased during spring and fall 



sources for higher trophic levels, indicating that 
cyanobacteria production was a sink, not a link, in the 
food chain. 

With the re-introduction of fresh water, more oxidized 
forms of nitrogen were introduced into the delta. 
Because nitrate and nitrite are the preferred forms of 
nitrogen by diatoms, these organisms contributed a 
substantial proportion of the productivity. Flow events 
resulted in moderate levels of primary production (per 
unit volume), but the total amount of fixed carbon was 
very high because the large volumes of water involved 
when elevations approach a meter or so in depth. 
Because diatoms were significandy contributing to the 
total productivity, there was a link with higher trophic 
levels, and the grazing food chain was stimulated. 
Increased freshwater inflow lowered salinity 
concentrations in the soils and waters of the marsh, 
stimulating plant production as well. Marsh 
productivity ultimately drives the detrital food chain 
when the plant material dies and decomposes. 

The increased primary production, in both the water 
column and marsh, resulted in increased secondary 
productivity in several ways. Grazing organisms could 
direcdy utilize the benthic and plankton diatom 
production. Increased carbon fixation led to higher 
amounts of detritus and higher amounts of food 
available to detritivores. A multiplier effect was present 
because increased marsh vegetation also increased 
habitat quality and complexity. Marsh areas are 
important because they provide nursery habitats and 
for many commercially and recreationally important 
species, {e.g., shrimp, red fish, and sea trout). Without 



freshwater flow and concomitant increases in the 
marsh system, the habitat does not support these 
estuarine-dependent species. Consequendy, the 
combination of increased food and increased habitat 
quality and area likely resulted in geometric increases in 
secondary production. 



A CONCEPTUAL MODEL 

The data collected during the present study allowed the 
creation of the conceptual model presented in 
Figure 7-4 on how Rincon Bayou fimctions, and, 
perhaps more importandy, how the upper delta 
functions with and without freshwater inflow. Rincon 
Bayou's connection with Nueces Bay allows the 
exchange of materials and energy flow between the two 
bodies of water. In addition, nekton (fish and 
epibenthic shrimp) utilize Rincon Bayou as a nursery 
habitat when water elevations and salinity conditions 
are suitable. 

The water levels and salinity concentrations in Rincon 
Bayou are governed by the interactions between tide, 
rain, evaporation and freshwater inflow (Figure 7-4). 
These outside forcing elements also drive nutrient 
concentrations, primarily through tides and inflow. 
Nutrient concentrations are also governed by 
biogeochemical processes associated with 
decomposition of marsh grass, excretion by organisms 
and recycling of dead organic matter. The nutrients, 
with sunlight, drive primary production in the water 
column (phytoplankton), which in turn drives a grazing 



7-8 ^ Synthesis and Conclusions 




Figure 7-4: Conceptual model of the Nueces Delta ecosystem. Parts of the Nueces Estuary are bounded by shaded 
rectangles. Circles are forcing functions from outside the system. Rounded rectangles represent standing stocks or 
energy storage. Arrows represent flow rates of energy and materials. The diamond represents on/off switch. Pentagons 
represent multipliers. 



food chain. Some benthic invertebrates then utilize 
this marsh and water column detritus, contributing 
themselves to the detrital food chain. Marsh 
production is a function of sunlight, nutrients and soil 
salinity. The soil salinity is affected by water column 
salinity and elevation. All benthos are strongly affected 
by salinity because of physiological tolerance to a 
specific sahnity range, or by using salinity gradients as 
cues for migration and reproduction. Of these 
elements, fresh water is the most critical for 
maintaining ecological functions in the upper Nueces 
Delta. In natural systems, inflow acts as a continuous 
forcing function. In this conceptual model, a diversion 



project acts as a "switch" that, when activated, restores 
some freshwater inflow to the system. 

This conceptual model demonstrates an improved 
understanding of the mechanisms controlling 
production in the upper Nueces Delta. These 
mechanisms indicate the importance of the 
demonstration project in restoring fiinctionality to the 
marsh-dominated ecosystem. Without fresh water 
from river diversions, salinity concentrations increase, 
average water levels decrease, and production of 
primary and secondary producers declines. A loss of 
fiinctionality in the Nueces Delta affects Nueces Bay 



Chapter Seven ^ 7-9 



because of the reduced transport of exported materials 
and the loss of habitat needed by migratory and 
nursery-dependent species that utili2e the delta. 
Therefore, without freshwater inflow, the delta is not a 
functioning component of the greater Nueces Estuary 
ecosystem. 



SUMMARY 

Since 1982, the average annual amount of freshwater 
inflow to the upper delta has decreased by over 99% 
compared to the period before 1958 (Irlbeck and Ward 
2000). This dramatic change indicates the large degree 
to which human activity has altered the delta ecosystem 
in a relatively short period of time {i.e., less than a 
quarter century). Nevertheless, the demonstration 
project successfully increased the amount of fresh 
water diverted into the upper Nueces Delta by six or 
seven times that which would have occurred without 



the project. Although this amount of restored inflow 
was relatively small (only about 2% of the annual 
average before 1958) when compared to historical 
volumes, it returned a significant degree of ecological 
function to the Nueces Delta and Nueces Estuary 
ecosystems. Prior to the demonstration project, 
persistendy high salinity concentrations severely 
inhibited the function of the delta, and its natural 
contribution to the greater estuary ecosystem was 
limited to infrequent periods when natural flow events 
occurred. With the restored regular interaction 
between the river and Rincon Bayou, fresh water and 
nutrients were more consistendy introduced into the 
upper delta, stimulating critical chemical and biological 
processes. As a result, habitat in the delta component 
of the Nueces Estuary improved in both quality and 
quantity, and foraging opportunities for many estuarine 
species were increased. 



7-10 ♦♦♦ Synthesis and Conclusions 



CHAPTER EIGHT 

Future 
Opportunities 



"In nature there are neither rewards nor 
punishments - there are consequences." 

♦♦♦ R.G. Ingersoll 



The Rincon Bayou Demonstration Project was a short- 
term initiative undertaken to answer two fundamental 
questions about the Nueces Delta ecosystem. First, 
"Was there an opportunity to meaningfully increase the 
amount of fresh water diverted into the upper delta?", 
and second, "Would the biological resources in the 
delta measurably respond to increased fresh water in a 
favorable way?" The demonstration project answered 
the first question with a definitive "Yes." Data indicate 
that project features increased the total amoiont of 
diverted fresh water by 732% during the demonstration 
period (Chapter 3). In the long-term, had the 
demonstration project features been in place since 
1982, the average annual amount of freshwater 
diversion into the upper delta would have been 
increased by 633% (Irlbeck and Ward 2000). Project 
results also answered the second question with an 
equally definitive "Yes". Because of the freshwater 
diversions made by the demonstration project, the 
"reverse esmary" salinity gradient that had previously 
characterized Rincon Bayou and the upper delta 
reverted to a more natural pattern (Chapter 7). Primary 
productivity in the water column was stimulated 
(Chapter 4), and the abundance, diversity and 
distribution of both benthic and emergent vegetation 
communities increased as a result of additional fresh 
water (Chapters 5 and 6). In summary, the 
demonstration project was more successful than had 
been anticipated, both in the amount of fresh water 
diverted and in the biological responses observed. 

Because of limitations in program authority and 
landowner agreements, the Rincon Bayou 
Demonstration Project was conducted as only an 
investigation of the potential for restoration of natural 
estuary function. Based upon project results, actual 

Chapter Eight ♦ 8-1 



function can be restored. The next step in restoring 
freshwater flow to the upper Nueces Delta should be 
the implementation of a long-term (permanent) 
diversion project. In the context of this future 
scenario, the authors of this report present several 
opportunities for enhanced project design and future 
ecological studies. These recommendations were 
developed based upon observ^ations gained from the 
results of the demonstration project. 



OPPORTUNITIES FOR A 
PERMANENT DIVERSION PROJECT 

From analysis of the hydrographic data collected during 
the demonstration period, a permanent diversion 
project could be designed that would produce 
hydrographic benefits exceeding those of the 
demonstration project. During the demonstration 
period, the primary limitations of the total volume of 
freshwater diverted during a given hydrographic event 
were restrictions imposed by channel capacity and 
channel obstructions. Widening some reaches of 
Rincon Bayou in the upper delta that have significant 
restrictions in channel capacity {e.g., the reach between 
the Nueces Overflow Channel and the low water 
crossing at the head of Rincon Bayou) would result in a 
greater available cross-sectional area and lower 
factional resistance. 

Also, if existing channel obstructions {e.g., the private 
road crossing separating the upper and central Rincon 
Bayou segments, and the remaining fill material in the 
north end of the Rincon Overflow Channel) were 
removed, diversion rates during events would improve 
(Bureau of Reclamation 2000). Also, the amount of 
water passing out of the delta back into the river 
through the diversion channel(s) at the end of a given 
hydrographic event would likely be reduced. Each of 
these two recommendations would increase the 
potential for freshwater diversions beyond that 
provided by the demonstration project design. 

As with the short-term demonstration, a long-term 
diversion project would have to address the issue of 
voluntary landowner participation. This factor 
prevented the continuation of the demonstration 



project. Although key landowners were wiUing to 
consider easements that would have allowed project 
features to remain in perpetuity, a price agreeable to all 
parties was not able to be negotiated. 



OPPORTUNITIES FOR FURTHER 
ECOLOGICAL STUDY 

Selection and Monitoring of 
Indicator Species 

The use of one (or a few individual) species to reflect 
the overall condition of an ecosystem is not a new 
concept. For such indicator species to be useful in 
applied research, they should have the following 
characteristics: 1) they should direct attention to 
qualities of their environment, 2) they should give an 
indication that some environmental characteristic is 
present, 3) they should express a generalization about 
their environment, 4) their study should suggest a 
cause, outcome or remedy, and 5) they should show a 
need for action (Soule 1988). During the 
demonstration period, benthic organisms were useful as 
indicators of ecosystem productivity, particularly in 
regards to the effects of freshwater inflow. 

Benthos 

Future monitoring of delta productivity should 
consider the use of indicator benthic invertebrates. 
These organisms are useful indicators because they are 
relatively long-lived and sessile, so they integrate the 
effects of freshwater inflow over appropriate temporal 
and spatial scales. Changes in benthic biomass and 
abundance indicate changes in secondary productivity, 
and changes in benthic biodiversity are an important 
indicator of habitat quality. Although the 
demonstration project focused primarily on infauna 
{i.e., animals living within sediments), an important 
aspect of improving inflow conditions in the Nueces 
Delta is restoring habitat functionality. This issue could 
be assessed using epifauna {i.e., animals living on or 
near the sediment surface) as well. Abundances of 
epifaunal organisms (i?.^., shrimp, crabs, moUusks and 
benthic feeding fish) would indicate habitat utilization. 



8-2 V Future Opportunities 



Because these organisms are highly seasonal, monthly 
sampling of epifauna would likely be required. 

Vegetation 

Certain vegetation species have also proved to be 
useful indicators of the timing and quantity of 
freshwater inundation needed to promote sexual 
reproduction and plant expansion in hypersaline 
marshes. Annual species like Salicomia bigelovii, for 
which successful establishment can only occur if soil 
salinity concentrations are reduced to a level that 
alleviates the osmoticaUy induced seed dormancy, could 
provide relevant information regarding the timing and 
quantity of fresh water needed to promote sexual (seed) 
colonization in hypersaline salt marshes. Because this 
species occurs only after significant freshwater 
inundation events during the fall and early winter, 
spring-time biomass samples may be useful in 
indicating relative plant productivity between the 
stations. 

During the demonstration project, seasonal changes in 
emergent vegetation cover and biomass were measured 
and correlated to overall delta productivity in response 
to freshwater inflow. However, several changes in the 
sampling procedure could significandy contribute to 
future monitoring. These changes include the addition 
of shorter and more closely spaced transects, more 
detailed sampling of pore water salinity concentrations 
and focus on the colonization of opportunistic species 
following major precipitation and inflow events. 
Primarily, additional vegetation transects should be 
established in several places located direcdy on Rincon 
Bayou. While sampling in the tidal flats near the 
Rincon Overflow Channel {i.e.. Station II) indicated 
changes after major flow events, vegetational changes 
direcdy along the upper portions of Rincon Bayou 
likely occurred after freshwater flow through the 
Nueces Overflow Channel during relatively smaller 
positive-flow events. A useful approach would be to 
sample four transects all on Rincon Bayou, with the 
first transect being located close to the Nueces 
Overflow Channel and the fourth transect being near 
Nueces Bay. Shorter transects with closer sampling 
lines may provide a more detailed picture of vegetation 
changes {e.g., 50-m long transects with sampling lines 



spaced 2-m apart). Most importandy, every effort 
should be taken to ensure that salinity measurements 
are acquired on each sampling date. Ideally, pore water 
salinity measurements would be taken at 10-m intervals 
(if a shorter transect were used) rather than 
50-m intervals, as accurate and complete salinity 
measurements are key to understanding the effects of 
firesh water and consequent changes in ■vegetation. 



Modeling 

The demonstration monitoring program documented 
changes in biological productivity and species 
composition in relation to the alteration of the 
fireshwater inflow regime. There is an opportunity to 
integrate the various data components of this study to 
determine; 1) how the marsh would have responded 
during the demonstration period without project 
diversions, and 2) how the marsh ecology would 
respond to different fireshwater inflow conditions 
Considerably more data would need to be collected to 
provide these answers through field studies. 

A numerical model could be developed to calculate 
productivity changes in response to prescribed inflow 
events. One such modeling concept {e.g., the 
conceptual model presented in Chapter 7) has already 
been outlined. Once developed, this model could be 
used to stmidate productivity in Nueces Delta with and 
without the demonstration project by using the existing 
monitoring data to calibrate the model. The change in 
productivity with and without freshwater inflow would 
allow calculation of the percent change due to the 
observed restored flow volume. This change would be 
a direct estimate of the benefits of the demonstration 
project. Furthermore, the modeling of other fresh 
water input scenarios could be used to estimate the 
benefits of particular permanent diversion project 
designs. 

A numerical model would also improve the 
understanding of how the marsh functions under 
various conditions. Model sensitivity studies could 
determine transition points within the ecosystem, as 
well as how to maximize benefits with adjustments in 
the timing and amount of freshwater inflow. 



Chapter Eight ♦ 8-3 



Additionally, this model, with adequate built-in 
generality, could be applied to other estuary systems 
with similar characteristics and freshwater allocation 



OPPORTUNITIES FOR 
INTEGRATION WITH BAY AND 
ESTUARY RELEASE SCHEDULES 

At present, the City of Corpus Christi is required to 
make pass-through releases of water from the reservoir 
system on a monthly basis for bay and estuary needs. 
Because of the high flooding threshold of north bank 
of the Nueces River, none of this water direcdy reaches 
the upper delta. Only with the demonstration project, 
which allowed for a regular {i.e., daily) exchange of 
small volumes of water between the river and Rincon 
Bayou, was released water able to (occasionally) freshen 
the upper delta. Information gained from the 
demonstration project indicates that &esh water passing 
through Rincon Bayou provides a more direct benefit 
to the estuary ecosystem than water by-passing the 
Nueces Delta and flowing direcdy into Nueces Bay. 
Therefore, this finding suggests an opportunity for 
integrating a permanent diversion project with reservoir 
operations. 

Demonstration data suggest that the Nueces Estuary 
would benefit more if freshwater releases could be 
made in such a way as to trigger positive- flow events 
into Rincon Bayou and the upper delta. It was 
observed during the demonstration period that flow 
events coincident with elevated water levels in Nueces 
Bay caused a greater proportion of fresh water to be 
diverted into the upper delta (Ward 2000). Ward 
(1997) and Chew (1964) have indicated that seasonal 
secular excursions in the Gulf of Mexico, which are 
well reflected in water level variations in the upper 
delta, are Likely during the spring and autumn, although 
with varying magnitudes and durations. There was also 
an observed seasonality to the ecology of the Nueces 
Delta. Animal recruitment and marsh plant growth 
occurred in spring, and nursery habitat utilization in 
fall. Therefore, given the combined probability of 
higher water levels in Nueces Bay and increased 
ecological benefits to living resources during the spring 



and fall seasons of the year, larger, quarterly (or 
possibly semi-annual) releases from the reservoir 
system could be more direcdy beneficial to the delta 
ecosystem than smaller, monthly releases. 

There are also theoretical reasons why bigger, less 
frequent inflow events would be more beneficial. An 
emerging paradigm suggests that large "pulsed," or 
punctuated, events will favor large phytoplankton that 
can out-compete small phytoplankton for nutrients 
when they are present at elevated concentrations (Sutde 
et al. 1988). Therefore, a pulsed nutrient supply wiU 
select for larger phytoplankton, which can out-compete 
smaller phytoplankton (Turpin and Harrison 1980; 
Sutde et al. 1987). This results in a food-web based on 
large-size phytoplankton, which is much more efficient 
in transferring nutrients and energy to higher trophic 
levels than is a food-web which is based on pico- or 
nano-plankton. For example, a simple model based on 
empirical data indicates that distributing nitrogen in 
pulses rather than at a low homogeneous concentration 
results ia 1.5 times more carbon in large zooplankton 
than would occur if the nutrients were present at a low 
homogeneous concentration (Sutde et al. 1990). The 
results for phosphate are even more dramatic, where 
there would be 3.6 times more carbon in large 
zooplankton under a pulsed delivery regime. These 
results suggest that releasing water ia large pulses rather 
than in a continuous manner may deliver more 
necessary resources to fish and other larger consumers 
in Nueces Estuary. 



OPPORTUNITIES FOR ADAPTIVE 
MANAGEMENT 

Incorporating demonstration project features into a 
permanent diversion project, modifying reservoir 
operations, and continuiag to smdy resultant biological 
responses in the Nueces Delta would present a unique 
opportunity for one of the most comprehensive studies 
of ecological benefits accrued by adaptive management. 
Were such an endeavor to be undertaken, four 
fundamental questions should be addressed: 



8-4 ♦♦♦ Future Opportunities 



1) Which delivery schediile provides more benefit to 
estuary productivity: pulsed or continuous? 
Included in this question would be a determination 
of the release volume necessary to trigger a delta 
diversion event. 

2) How far downstream in Rincon Bayou do 
beneficial effects accrue? That is, the idea that 
there is a functional linkage between the marsh, 
delta, and bay should be tested explicidy. 

3) Does export from the marsh benefit the bay, and 
how much water is necessary for this benefit to 
accrue? There is Utde doubt of an existing linkage 
between the marsh and the bay, but it is not clear 
what volume of fresh water is necessary to 
maintain a functional linkage. 



4) What are the specific trophic linkages between 
marsh, planktonic, and benthic production, and 
how do these resources affect production of 
commercially and recreationally important species 
{e.g., shrimp, fish and wildlife). 

The minimum freshwater flow necessary for 
maintaining the ecological integrity of bay and estuary 
ecosystems is an emerging issue in water resources 
management, nation-wide. The complexity of this issue 
is further magnified in estuary systems that are 
supported by a semi-arid watershed and located 
adjacent to a large metropolitan area. Therefore, the 
Nueces Delta and Estuary is an ideal place to develop 
definidve answers to the question of how to most 
effectively allocate limited freshwater resources. 




Figure 8-1 : View of the lower Nueces Delta with the City of Corpus Christi in the background. 



Photo courtesy of the Bureau of Reclamation. 



Chapter Eight ♦ 8-5 



CHAPTER NINE 

Literature Cited 



CHAPTER 1: INTRODUCTION 

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Texas. 

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Longley, W. L., ed. 1994. Freshwater inflows to Texas 
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Montagna, P.A., S. Holt, C. Hitter, K. Binney, S. 

Herzka, and K. Dunton. 1998. Characterization 
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Austin, Texas. 



Chapter Nine ♦ 9-1 



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CHAPTER 2: STUDY AREA 

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9-2 ♦ Literature Cited 



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Chapter Nine ♦ 9-3 



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CHAPTERS: HYDROGRAPHY 



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and Mission- Aransas estuaries: A study of the 
influence of freshwater inflows. Texas 
Department of Water Resources, LP- 108, Austin, 
Texas. 

Ward, G.H. 1997. Processes and trends of circulation 
within the Corpus Christi Bay National Estuary 
Program Study 7\j:ea. Report CCBNEP-21, 
Corpus Christi Bay National Estuary Program, 
Corpus Christi, Texas. 

Ward, G.H. 2000. Hydrography of the Nueces Delta 
and Esmary: 1992-1999. Unpublished. In 
Concluding Report: Rincon Bayou Demonstration 
Project, Appendix B. United States Department of 
the Interior, Bureau of Reclamation, Austin, Texas. 



Bureau of Reclamation. 2000. Hydraulic Analysis of 
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Enhancement Project. United States Department 
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Regional Office, Billings, Montana. 

Irlbeck, M.J. and D. Ockerman. 2000. Technial notes 
on the Rincon gauge and data. Unpublished. In 
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Project, Appendix A. United States Department 
of the Interior, Bureau of Reclamation, Austin, 
Texas. 

Irlbeck, M.J. and G.H. Ward. 2000. Analysis of the 
historic flow regime of the Nueces River into the 
upper Nueces Delta, and of the potential 
restoration value of the Rincon Bayou 
Demonstration Project. Unpublished. In 
Concluding Report: Rincon Bayou Demonstration 



CHAPTER 4: WATER COLUMN 
PRODUCTIVITY 

Boynton, W.R., W.M. Kemp and C.W. Keefe. 1982. 
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production. In Estuarine Comparisons. 
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CHAPTERS: BENTHIC 
COMMUNITIES 

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337-345 
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seagrass meadows: '^C evidence for the 

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& Sons, Inc., New York. 257 p. 
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Martin, CM. and P.A. Montagna. 1995. 

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Montagna, P.A. 1995. Rates of meiofaunal 

microbivory: a review. Vie et Milieu 45:1-10. 

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185:149-165. 

Montagna, P.A. and R.D. Kalke. 1992. The effect of 
freshwater inflow on meiofaunal and macrofaunal 
populations in the Guadalupe and Nueces 
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Montagna, P.A. and R.D. Kalke. 1995. Ecology of 
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American Malacological Bulletin 11:1 63- 175. 

Montagna, P.A., D.A. Stockwell and R.D. Kalke. 1993. 
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populations and feeding during the Texas brown 
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Montagna, P.A. and W.B. Yoon. 1991. The effect of 
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Nixon, S.A., C.A. Oviatt, J. Frithsen and B. Sullivan. 
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Ritter, M.C. and P.A. Montagna. 1999. Seasonal 

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Ritter, M.C. and P.A. Montagna. 2000. Effects of 
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CHAPTER 6: VEGETATION 
COMMUNITIES 

Adams, D.A. 1963. Factors influencing vascular plant 

zonation in North Carolina saltmarshes. Ecology 

44:445-446. 
Adam, P. 1990. Saltmarsh Ecology. Cambridge 

University Press. 
Allison, S.K 1996. Recruitment and establishment of 

salt marsh plants following disturbance by 

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Allison, S.K 1992. The influence of rainfall variability 

on the species composition of a northern 

CaUfomia salt marsh plant assemblage. Vegetatio 

101:145-160. 
Badger, KS. and I. A. Unger. 1990. Effects of soil 

salinity on growth and ion content of the inland 

haloph)rte Hordeum juhatum. Botanical Gat^tte 

15(3):314-321. 
Barbour, M.G. 1970. Germination and early growth 

of the strandline plant Cakile maritima. Bulletin of the 

Torrey Botanical Club 97:13-32. 
Barbour, M.G. and C.B. Davis. 1970. Salt tolerance of 

five California salt marsh plants. American Midland 

Naturalist 84: 262-265. 
Bertness, M.D. and A.M. Ellison. 1987. Determinants 

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community. Ecological Monographs 51 (7): 129-147. 
Bertness, M.D., L. Gough and S.W. Shumway. 1992. 

Salt tolerances and the distrubution of fugitive salt 

marsh plants. Efo%73(5):1842-1851. 



9-6 V Literature Cited 



Broome, S.W., W.W. Woodhouse Jr., and D.E. Seneca. 
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The effects of N, P, and Fe fertili2ers. Soil Sciences 
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Brugnoli, E., and O. Bjorkman. 1992. Growth of 
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Burkholder, P.R. and L.M. Burkholder. 1956. Vitamin 
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Chapman, V.J. 1974. Salt marshes and salt deserts of 
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Clewell,A.F. 1997. Vegetation. Pages 77-109 /« 

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Covin, J.D. andJ.B. Zedler. 1988. Nitrogen Effects 
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De la Cruz, A.A., C.T Hackney and J.P. Stout. 1979. 
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Dunton, K.H., B. Hardegree, and T.E. Whidedge. 
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pulse on the growth and elemental composition of 
natural stands o( Spartina altemiflora ^nd] uncus 
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Irlbeck, MJ. and G.H. Ward. 2000. 7\nalysis of the 
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upper Nueces Delta, and of the potential 
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Longley, W.L., ed. 1994. Freshwater inflows to Texas 
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Mendelssohn, I. A. 1979. The influence of nitrogen 
level, form, and application method on the growth 
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Odum, H.T. and R.F. Wilson. 1962. Further studies 
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Parsons, T.R., Y Malta, and CM. LaUi. 1984. A 
manual of chemical and biological methods for 
seawater analysis. Pergamon Press, New York, 
New York. 

Phelger, C.F. 1971. Effect of salinity on growth of a 
salt marsh grass. Ecology 52:908. 



Chapter Nine ♦ 9-7 



Philipupillai, J., and I. A. Unger. 1984. The effect of 
seed dimorphism on the germination and survival 
of Salicomia europaea L. populations. American 
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Riehl. T.E. and I.A. Ungar. 1982. Growth and ion 
accumulation in Salicomia eurpaea under saline field 
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Teal, J. M. 1962. Energy flow in the salt marsh 
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Ungar, I.A. 1962. Influence of salinity on seed 
germination in succulent halophytes. Ecology 
43:763-764. 

Ungar, I.A. 1974. The effect of salinity and 

temperature on seed germination and growth of 
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1362. 

Ungar, I.A. 1978. Halophyte and seed germination. 
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Ungar, I.A. 1991. Ecophysiology of vascular 
halophytes. CRC Press, Boca Raton, FL. 

Ungar, I.A. 1995. Seed germonation and seed-bank 
ecology in halophytes, p. 599-628. In]. Kigel and 
G. Galili (eds.), Seed development and 
germination. Marcel Dekker, Inc, New York, 
New York. 

Valiea, I., andJ.M. Teal. 1974. Nutrient limitation in 
salt marsh vegetation, p. 547-563. In R.J. Reimold 
and W.H. Green (eds.), Ecology of halophytes. 
Academic Press, New York, New York. 

VaHea,I.,J.M.Teal,andW.G.Deuser. 1978. The 
nature of growth forms in the salt marsh grass 
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Weilhoefer, C.L. 1998. Effects of freshwater inflow, 
salinity and nutrients on salt marsh vegetation in 
South Texas. Thesis. University of Texas at 
Austin. 

Yeo, A.R. 1983. Salinity resistance: physiologies and 
prices. Physiologia Plantaraum 58: 214-222. 

Zedler,J.B. 1983. Freshwater impacts on normally 
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Zedler,J.B. andP.A. Beare. 1986. Temporal 

variability of salt marsh vegetation: the role of low- 
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306 in D. A. Wolfe, ed. Estuarine Variability. 
Academic Press, Inc. Orlando, Florida. 



CHAPTER?: SYNTHESIS AND 
CONCLUSIONS 

Costanza, R., R. d'Arge, R. de Groots, S. Farber, M. 
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Robinson, L., P. Campbell and L. Buder. 1995. 
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the Interior, Bureau of Reclamation, Austin, Texas. 



CHAPTERS: FUTURE 
OPPORTUNITIES 

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Rincon Bayou Using HEC-2, Rincon Bayou- 
Nueces Marsh Wedands Restoration and 
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9-8 ♦ Literature Cited 



Irlbeck, M.J. and G.H. Ward. 2000. Analysis of the 
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restoration value of the Rincon Bayou 
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Project, Appendix B. United States Department of 
the Interior, Bureau of Reclamation, Austin, Texas. 



Chapter Nine ♦ 9-9 



•«i«$gM«p»- 






Appendices 











< 




Contents 



Page 

A-1 A Technical Notes ON THE RiNCON Gauge AND Data 

B-1 B Hydrography OF THE Nueces Delta AND ESTUARY: 
1992-1999 

C-1 C ANALYSIS OF THE Historic Flow REGIME OF THE Nueces 

RrVER INTO THE UPPER NUECES DELTA AND OF THE 
POTENTLY. RESTORATION VALUE OF THE RiNCON BAYOU 
DEMONSTRATION PROJECT 

D-1 D RECENT Trends IN Precipitation Occurring ON THE 
Nueces River Watershed of South Texas 

E-1 E utilization OF EsTUARiNE Organic Matter During 
Growth and Migration by Juvenile Brown Shrimp 

PENAEUS AZTECUS IN A SOUTH TEXAS ESTUARY 
F-1 F EFFECTS OF TEMPORALTTY, DISTURBANCE FREQUENCY AND 

Water Flow on an Upper Estaurine Macroinfauna 
Community 

G-l G FIELD Notes AND Observations FROM Benthic Sampling 
Trips: October 1994 -December 1999 



APPENDIX A 



Technical Notes on the Rincon 
Gauge and Data 



Michael J. Irlbeck 
Darwin Ockennan 



U.S. Bxireau of Reclamation 
United States Geological Survey 



INTRODUCTION 

On May 16, 1996, as part of the Bureau of Reclamation's (Reclamation) Rincon Bayou Demonstration Project, 
the U.S. Geological Survey (USGS) installed a stream flow gauging station on the head-water channel of Rincon 
Bayou (Station 08211503, Rincon Bayou Channel Near CalaUen, Texas) located just downstream from the 
Nueces River Overflow Channel (Figure 1). The purpose of this gauge was to record daily stage and discharge 
through the overflow channel, and direct precipitation at the site during the demonstration period. Data from 
the Rincon Gauge was available from the date of its installation (May 16, 1996) through December 31, 1999. 




Figure 1: Location of the Rincon gauge. 



AppendixA ♦ A-1 



Instrumentation 

All data ftom the gauge was measured in 
1 5-minute intervals, stored by a data collection 
platform (DCP), and transmitted to USGS offices 
via GOES (Geosyn-chronous Operational 
Environmental Satellite) in near real time 
(Figure 2). Water level ia the channel was 
measured by a pressure transducer, and the 
gauged water level {i.e., gauge height) was 
referenced to mean sea level. Datum of the gauge 
is at mean sea level. Row velocity was measured 
by an acoustic velocity meter deployed near the 
center of the channel (Figure 3). The acoustic 
meter had a resolution of 0.01 feet per second and 
measured both positive and negative flow in the 
channel. Rainfall was measured by a tipping 
bucket rain gauge. Discharge in the channel was 
computed as the product of the flow cross- 
sectional area and the mean chatmel velocity. 




Figure 2: View of the 
Data Collection Platform, 
Rincon gauge. 



Figure 3: View of the 
gauging instrumentation, 
Rincon gauge. 



At this site, there is no well-defined relation between water level (stage) and discharge due to the effect of tide. 
The cross-sectional area of flow in the channel is a function of water level in the chaimel. The relation between 
water level and cross-sectional area was therefore determined from a cross-section elevation survey. 
Furthermore, the acoustic velocity meter measures flow velocity at a single point in the channel. Because the 
measured velocity is not necessarily the mean velocity in the channel, manual discharge measurements were 
made to determine discharge and mean channel velocity. These measurements of mean channel velocity were 
related to acoustic velocity measurements by a rating developed from the manual discharge measurements. 
When actual measured discharge (during calibration measurements) were compared with discharge values 
determined from calibrated gauge readings, the potential error for flows above 1 cfs were usually within about 
10 percent. 

SUMMARY OF DATA 

Origirjally, the Nueces Overflow Channel was designed to have a controlling bottom elevation of 2.0 ft msl. 
However, days after construction of the channel was completed in late October 1995, the Corpus Christi area 
received 10-12 inches of local ratafaU in a 2-day period. The resulting runoff and river discharge scoured the 
newly cut channel to a new controlling bottom elevation at about mean sea level. The effect of this change was 
that, in addition to freshwater discharge events through the overflow channel, there was also now the 
opportunity for regular tidal exchange between the river and the upper delta, even when there was litde or no 
flow coming down the Nueces River. As a result, flow in the channel regularly occurred in both directions 
during the study period. Positive flow from the Nueces River into Rincon Bayou typically occurred during 
periods of high discharge in the Nueces River or during rising tide events which pushed water up the river and 
through the overflow channel. Negative flow from Rincon Bayou into the Nueces River typically occurred 
when the water level in the upper delta was relatively high immediately after river discharge events or during 
falling tide conditions. 

Stage 

As discussed above, the Rincon gauge is tidally influenced. The stage data recorded by the gauge therefore 
indicate influences by both freshwater flow events {i.e., stage events driven by discharge in the Nueces River) 
and saltwater inundation events {i.e., stage events driven by tidal or other hydro-meteorological activity). This 
dual relationship can be best shown by comparing stage values between the Rincon gauge and the Calallen 
gauge (Station 08211500, Nueces River at Calallen) (Figure 4). Although there is a strong correlation between 
the two gauges, the variance in daily stage at the Rincon gauge is about 1.5 to 2.0 ft for any given stage value at 
Calallen. This variance is the result of tidal mfluences on the upper delta and the Nueces River below Calallen, 
which is present regardless of flow in the river. 



A-2 ^ Technical Notes on the Rincon Gauge and Data 



The tnaviimiiTi stage recorded at the Rincon gauge during a large discharge event in the Nueces River was 
7.38 ft msl on October 21, 1998. The maximum stage recorded during a tidal event not influenced by the 
Nueces River was 5.35 ft msl on August 23, 1999, which was associated with the initial storm surge of 
Hurricane Bret. A summary of the stage data collected during the study period is displayed in Figure 5 
(monthly) and Table 1 (daily). 



Table 1: Summary of Daily Stage Data (ft msl), Rincon Gauge. May 16, 1996 - December 31, 1999. 



Stage 



Jan Feb 



Mar 



Apr May 



Jun 



Jul 



Aug Sep 



Oct 



Nov Dec 



Maximum 1.62 2.31 2.89 3.30 

Minimum 1.03 0.97 0.97 1.11 

Average 1.25 1.43 1.65 1.89 



3.10 5.63 5.34 4.52 5.72 7.25 3.49 1.96 
1.32 1.25 1.00 1.08 1.25 1.18 1.05 0.97 
1.80 1.83 1.90 1.59 2.18 2.55 1.79 1.26 



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ir 








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Daily Stage at Calallen (ft) 



Figure 4: Plot of daily stage values from the Calallen and Rincon gauges. May 16, 1996, 
through December 31, 1999. 



Appendix A ♦ A-3 




n r 

Jan Feb Mar Apr May Jun 



Aug Sep Oct Nov Dec 



Figure 5: Average monthly stage, Rincon Gauge. May 16, 1996 - December 31, 1999. 



Discharge 

As with stage, discharge past the Rincon gauge (either positive or negative) was determined by either freshwater 
flow in the Nueces River or tidal events in the bays, or both. Freshwater flow events were typically infrequent, 
of high magnitude and positive (i.e. into the delta), while tidally-driven discharge events were more frequent, of 
lesser magnitude and both positive and negative in direction. However, such a clear distinction between factors 
affecting discharge through the overflow channel is over simplified. This is because tidal conditions in the 
lower river and delta consistentiy exerted a strong influence on the rate of discharge, even during moderate flow 
events in the Nueces River. For example, daily flow in the Nueces River did not meaningfully contribute to 
discharge through the Nueces River Overflow Channel when below about 650 cfs, and did not become the 
dominant factor until river flow exceeded approximately 1,400 cfs (Figure 6). For flow values in the Nueces 
River below 1,400 cfs, the tide condition at the point of diversion was the dominant factor in determining both 
the direction and rate of discharge through the overflow channel. 

The largest daily discharge event associated with freshwater flow in the river was 274 cfs on October 21, 1998. 
The largest daily discharge event associated with a tidal event essentially independent of the Nueces River 
occurred on August 23, 1999, and was approximately 90 cfs. This discharge event was also associated with the 
initial storm surge of Hurricane Bret. A summary of the discharge data collected during the study period is 
displayed in Figure 7 (monthly) and Table 2 (daily). 



A-4 V Technical Notes on the Rincon Gau^ and Data 



Table 2: Summary of Daily Discharge Data (cfs), Rincon Gauge. May 16, 1996 - December 31, 1999. 



Discharge 


Jan 


Feb 


Mar 


Apr 


May 


Jun 


Jul 


Aug 


Sep 


Oct 


Nov 


Dec 


Maximum 


2 


9 


21 


36 


29 


121 


99 


90 


177 


274 


22 


3 


Minimum 





-2 


-9 


-13 


-14 


-12 


-69 


-42 


-60 


-66 


-16 


-1 


Average 


0.1 


0.2 


0.4 


0.6 


-0.4 


2.8 


3.6 


0.0 


6.2 


11.2 


1.0 


0.0 



Precipitation 

Precipitation in the study area was sporadic, but generally coincided with the spring (March through May) and 
late summer/ fall (August through November) seasons. The summer month of July and the winter months of 
December and January were consistently dry. A s umm ary of the precipitation data collected during the study 
period is displayed in Figure 8 (monthly) and Table 3 (daily). Specific precipitation events are considered in 
context with the discussions of significant discharge events. 



Table 3: 


Summary of Daily Rainfall Data (inches), Rincon Gauge. 


May 16, 


1996- 


December 31, 1999. 




Rainfall 


Jan 


Feb 


Mar 


Apr 


May 


Jun 


Jul 


Aug 


Sep 


Oct 


Nov 


Dec 


Maximum 0.35 
Average 002 


1.49 
0.06 


2.79 
0.10 


2.44 
0.08 


1.65 
0.06 


1.70 
0.09 


0.81 
0.03 


3.38 
0.15 


2.15 
0.10 


4.72 
0.17 


1.39 
0.06 


0.89 
0.02 



Appendix A ♦ A-5 



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500 1000 1500 

Daily Discharge at Calallen (cfs) 



2000 



2500 



Figure 6: Plot of daily discharge values from the Calallen and Rincon gauges. May 16, 1996, 
through December 31 , 1999. Flow in the Nueces River did not become the dominant factor 
determining the rate of discharge through the Nueces River Overflow Channel until it exceeded 
approximately 1 ,400 cfs. 



A-6 ^* Technical Notes on the Rincon Gauge and Data 



2500 



2000 





1— 
o 

-2- 

x: 
o 



1500 



1000 



S 500 - -■ 





- 199 
■ 199 

- 199 


6 

7 
8 
9 - 






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i ; ' ■ I 

1 \ 1 i 1 1 1 i i 



Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 



Figure 7: Total monthly discharge, Rincon Gauge. May 16, 1996 ■ 
December 31, 1999. 



AppendixA ♦ A-7 



O 

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q 

3 
■g. 
'o 

Q. 

O 



11 
10 

9 

8 

7 -I 

6 

5 

4 

3 

2 

1 



- 1996 

-• 1997 

— 1998 

• 1999 




-I — 1 1 1 I 1 1 r- 

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 



Figure 8: Total monthly rainfall, Rincon Gauge. May 16, 1996 - December 31, 1999. 



A-8 ♦♦♦ Technical Notes on the Rin(on Gauge and Data 



APPENDIX B 



Hydrography of the Nueces Delta 
and Estuary: 1992-1999 



George H. Watd Center for Research in Water Resources, University of Texas, Austin 

BACKGROUND 

Three time scales are of pertinence to the evaluation of hydrographic data in the Nueces Estuary: intratidal, 
intertidal, and event-duration. The intratidal (or intradiumal) time scale represents short-term behavior at an 
hourly resolution, the intertidal (or interdiumal) time scale represents day-to-day variations of hydrographic 
parameters averaged over 24 hours, and the event-duration scale extends over the time period encompassing all 
responses to a specific hydrographic event, and can range from several days to many weeks. 

Data Sources 

Hydrographic data for this analysis were obtained from a variety of sources, including the Texas Coastal Ocean 
Observing Network (TCOON) marine monitoring system of Texas A&M University-Corpus Christi Conrad 
Blucher Institute (CBI), the weather station network administered by the National Weather Service (NWS), and 
the national stream flow gauging program conducted by the United States Geological Survey (USGS) (Table 1). 

Table 1: Summary of hydrographic data sources. 



DATA SOURCE 



Parameter 



Measurement Locatior) 



Data Type 



Nueces River at Calallen, 
USGS 

TCOON system, CBI 



Corpus Christi Bay, NWS 



Rincon Bayou near Calallen, 
USGS 



Estimated daily flow 



Water level 

Wind direction and velocity 

Salinity 

Daily precipitation 



Water level 
Current velocity 
Calculated flow 

Daily precipitation 



Nueces River 



Nueces and Corpus Christi 
bays 



Corpus Christi 
International Airport 

upper Rincon Bayou 



Data recorded at 
15-minute intervals 

Data recorded as 
6-minute averaged 
values 

Data archived as daily 
values 

Data recorded at 
15-minute intervals 



Data archived as daily 
values 



Data obtained from the TCOON system included salinity and water level. For each of these parameters, hourly 
measurements were obtained from the CBI data archive, and for the analysis reported here, subjected to 
24-hour averaging to obtain daily mean values. The salinity data used were from the CBI SALT03 gauge, which 
is situated due south of White Point in the center of the bay, about equidistant from the mouth of Rincon 
Bayou and the mouth of the Nueces River, and therefore responds to flow from both conveyances. Salinity 
concentrations are measured by robot conductivity sensor and converted to salinity, reported in parts per 
thousand (ppt). 



Appendix B ♦ B-1 



Water level in Nueces Bay was that measured at CBI's White Point gauge, nearer the mouth of Rincon Bayou. 
While there is no doubt some slope to the water surface in Nueces Bay is in response to meteorology, tides and 
river hydrographs, this is negligible in comparison to the temporal excursions in water level in the river and 
marsh. Therefore, stage data from the White Point gauge was regarded as an acceptable indication of the 
general coincident elevation of Nueces Bay. 

The USGS gauges from which data were obtained included the Nueces River at Calallen (Station 0821 1 500) 
(Calallen gauge) and Rincon Bayou near Calallen (Station 08211503) (Rincon gauge). Flow data were obtained 
from both gauges, and stage and precipitation data were also obtained from the Rincon gauge. There are 
significant limitations to the accuracy of the flow data from the Calallen gauge at higher values for two primary 
reasons. First of all, the gauge ceases to represent the total flow of the Nueces River above about 56.63 m'/s 
(2,000 cfs) due to activation of additional flow channels in the floodplain. Second, no reliable field 
observations of discharge values are available above 77.87 m'/s (2,750 cfs), so all daily flow values in excess of 
77.87 m'/s (of which there were 3 in the record under review) were estimated by extrapolation. 

INTRATIDAL (hour to hour) ANALYSIS 

The Nueces Estuary and Delta systems are greatly affected by intratidal processes, including diurnal hearing and 
cooling, tidal inflows and outflows, and short term responses to meteorological forcing, such as sea breezes, 
convecrive storms, and frontal passages. Analysis of such short-term behavior provides insight into the 
dynamics of the study area and the cause-and-effect relations between hydro-meteorological events and the 
hydrographic response of the Nueces Delta. Most of the variation on this time scale is oscillatory, and 
therefore obsciues the longer period behavior. Intratidal-scale data are best depicted through continuous 
animated display. 

Time Series Display of Daily-Mean Hydrologicaland Hydrographic Data 

In the process of compiling data on the hydrographic elements of the Rincon Bayou Demonstration Project to 
support analysis of the various ecological aspects, it was noted that several of the Principal Investigators have 
data available from the Nueces Delta area dating back to the early 1990's, which might be of potential value as 
baseline information in assessing the response of the region to the diversion project. Therefore, the compiled 
data set included as much information that was available for the past decade. 

Prior to late- 1993, virtually the only extant data is salinity and inflow measured at Calallen. In 1993, rehable 
records from Conrad Blucher Institute (CBI) tide gauges and anemometers become available. By the date of 
the breaching of the Nueces Overflow Channel on 26 October 1995, fairly continuous data from the region is 
available. Only the USGS gauge in the Rincon chaimel itself is lacking, this not becoming operational until 
15 May 1996. 

The Nueces Delta in general is subjected to intradaily variations, in response to tides, wind events, water mass 
replacement, convecrive storms and other such short-term phenomena. Analysis of such short-term behavior 
provides insight into the dynamics of the region and the cause-and-effect relations between hydro- 
meteorological events and the hydrographic response of the Nueces marsh area. It is unlikely, however, that 
the ecological components respond on such short time scales. In any event, the biological and chemical 
observations that have been made in this project have been performed on longer time intervals. It is desirable 
to similarly depict the longer term, daily to weekly variations in the hydrographic environment to facilitate 
interpretation of the biological response. 

For this purpose, daily means have been computed for all of the hydrographic variables. For purposes of 
effective presentation, a computer display of this daily data has been constructed. Operation of the program 
"BAION," which should be installed and operated on a PC-type machine, is described below. 

Program Operation 

Two files were developed: 

BAION.exe 
IRLBECK.dai 

B-2 ♦ Hydroffophy of the Nueces Delta and Bstuaty: T 992-1 999 



These should be copied into the same directoty on your hard drive. The first is the actual program (i.e., 
executable), and the second is a flat-ASCII data file read by the program. Double-click on the BAION icon (or 
cUck on the "START" button on your WINDOWS toolbar, then "RUN...", then enter die path name for 
BAION in the dialog box). The MS-DOS window will appear displaying the BAION starting banner. 

At this point, the program will display a figure showing a circle embedded in a square. You may see an ellipse 
embedded in a rectangle. If so, you will need to adjust the display of your monitor, by using the controls on the 
rim of the monitor frame. (Many machines will retain these adjustments in memory, and automatically 
implement them whenever the same program is activated.) Program operation will resume when you press any 
key. 

The computer will ask for a starting date for the display. For now, you can simply press ENTER, and the 
display will start at the beginning of the record. Later, you can enter a specific date by using the format 
YYDDD. You will be prompted for any changes to the plotting scales for the display (answer "no" for now) 
and then you'll be prompted for assurance that the display should continue (answer "yes"). (The program is 
rather insecure, and needs frequent encouragement.) 

Now the data display wiU (hopefully) begin. AH controls on the program are effected by the keyboard. The 
bottom line of the display panel summarizes the controls available to the user: 

S - slows the rate of the display by inserting a delay between plotted points, can be pressed successive 

times to further slow the display (but see also "P") 
A - accelerates the rate of display by decreasing the time delay between plotted points 
P - "pause" or "point" display, holds the present display; each time "P" is pressed an additional point is 

displayed 
R - resumes the default display rate, can be used to cancel the effect of "P" 
X - refreshes the axes on the wind panel 
L - allows the user to re-scale displays while in progress 
Q - terminates the present display 

There are three panels in which various data are shown (Figure 1). The panel at the lower left indicates 
meteorological and astronomical controls. The daily-averaged vector-mean wind velocity is shown as a line 
element terminated by a small circle. (Arrowheads are too hard to plot.) The length of the line segment is 
proportional to the speed of the wind (the circle indicates 10 m/s) and the orientation of the segment is the 
direction to which the wind is flowing. In the example of Figure 1, the wind blows from SE to NW. 

Data from two locations in the region are shown. The yellow vector is representative of the north shore of 
Corpus Christi Bay. This is primarily the record from the CBI Ingleside anemometer, which is the earliest such 
record from the project region, beginning in late Jime 1992. Unfortimately, the data record terminates in 
December 1996. For the remainder of the period of display (through 31 December 1999), data from the 
Port Aransas anemometer is used. The combined record is therefore referred to as "North Bay." In July 1994, 
a red vector is added to the wind display: this is the data record from the Naval Air Station, characterizing the 
south shore of Corpus Christi Bay. There are frequent gaps in both anemometer records, so the redundancy of 
two data sources is useful. fThese also display the differing responses of the wind depending upon whether it 
blows over land or water.) 

The lower left panel also shows the lunar controls, depicting lunar aspect as a moving icon. The appearance of 
the icon shows phase of the moon (in Figure 1, the crescent). The vertical (y-) component of the position of 
the icon is the declination of the moon, and the horizontal (x-) component is an index of the proximit)' of the 
moon to the earth, the line marked "apog" corresponding to greatest distance ("apogee") and the line marked 
"perig" to smallest distance ("perigee"). The 12.4 and 24.8-hr tidal components are virtually eliminated by the 
24-hr averaging to which this data has been subjected. The lunar controls on ride are not, therefore, so obvious 
as when intradaily data are shown, but the lunar declination does account for some of the 15-day oscillation. 

Two other panels are shown on the right two-thirds of the screen. The lower of these displays water levels. 
There are three sites displayed: the CBI White Point tide gauge, the water level of Corpus Christi Bay, and the 
water level at the Rincon diversion USGS gauge. Corpus Christi Bay is the tide record from the CBI Aquarium 
gauge, with older records filled in from the CBI Ingleside gauges, vit^ 

Ingleside 92119-93268 

State Aquarium 93269 - 99365 



Appendix h ♦ B-3 



variable names Julian day (YYDDD) & date 

DISPtAY OF RINCON PRQ, 
'Upper plot: ~ White pt ^al Rincon sup^ 
ftower plot 
X White Point tide 
\cCBay tide 

Rincon gage stage 
Lunar declinat:j.6n 
wind I wind below, circle = 10 m/s 
vector .^ I DATE 98317 Nov 1 

apoq periq 



rainfall 



Calallen flow 



declination 
range 



Lunar icon 




salinity 

Flow in 
Rincon 
channel 



lunar 
^ ^efclination 



tide (stage) 
level 



pengee 
range 



tide datum 
end-of-month marker 



mnemonic line for key control options 



Figure 1 : Display Screen for BAION. 

All three stage records are referenced to the same ("arbitrary") consistent datum. (Also, the 24-hr mean values 
of the Rincon stage had to be re-computed to reference these to GMT, which is the averaging period for all of 
the other hydrographic data.) On this same panel, lunar declination is plotted as a faint blue covet. Although 
declination is indicated by the vertical position of the lunar icon on the left panel, it can be hard to follow in 
relation to the water level, requiring one eyeball to be fixed on the lower left panel while the other follows one 
of the traces in the right panel. The vertical lines demarcate months. 

The upper panel displays "hydrography", including the salinity variation m upper Nueces Bay, the super- 
elevation of Rincon gauge over White Point, and the measured discharge in the Rincon diversion channel 
(starting in May 1996), as well as Calallen flow and regional rainfall. The Rincon discharge is generally about 
10% of the flow at Calallen, so it is scaled tenfold to be plotted on the same axis (scale on the right side of 
panel). The salinity data is from CBI SALT03 gauge, which turns out to be situated due south of White Point, 
but in the center of the bay about equidistant from the moutli of the Rincon and the mouth of the Nueces. It 
therefore responds to flows in both conveyances. 

Daily rainfall is plotted as a light blue bar, positive downward from the top of the panel. This is a composite 
data set, consisting of the measiurement at the USGS gauge back to May 1996, and the Corpus Christi airport 
measurement prior to this. The Calallen flow is plotted as a bar graph at the "back" of the panel. This slows 
down the display, because more time is required to "paint" the record, but this strikes me as a clearer depiction 
of flow conditions. (Note the jazzy shadows of the data points when a trace crosses a flood hydrograph.) 

One problem with the record of super-elevation is that both the White Point and the USGS Rincon gauge peg 
at low water. The Rincon gauges is especially problematic. For these data, any day with more than 25% of the 
data (5 measurements) at the low pegged value was deleted from the record. For those days with 5 or less such 
pegged values, a daily mean was computed using the peg values. (To omit them would overestimate the mean 
stage.) Approximately 16% of the data at the Rincon gauge proved to be pegged values. Fortunately these 
were confined to winter or summer low flows, in the absence of significant river flow, so there should be littie 



B-4 ♦ Hydrography of the Nueces Delta and Estuaty: 1992-1999 



effect on the use of the data in the biological analyses. However, no super-elevation can be computed when 
one of the two stage values is missing, so these peg values become gaps in the super-elevation data stream. 

Comments on the Data 

upon starting the display at the beginning of the data period, vi^ 1 992, one is immediately struck by the 
substantial flow hydrographs in the Nueces River. A series of large hydrographs begin in February and 
continue through June, holding salinity concentrations to virtually fresh throughout this period and the early 
summer. The Interim Order mandated releases begin in September 1992, and are manifested as the small 
pvdses of inflow occurring near the end of each month. The 1992 fall high water occtirs in early October. 
During this period, the lunar declination is near its maximum attainable value, and only one front of any 
consequence occurs in the fall, the remainder of the period being dominated by onshore flow from the Gulf. 

In late June 1993, a substantial rainfall event over a three day period occurred. Simultaneously, a spike in water 
levels of over 0.5 m is registered synchronously in Nueces and Corpus Christi Bays. The cumulative rainfall in 
the event totaled 0.23 m as measured at Corpus Christi. Evidentiy, such a large volume of rainfall can account 
for at least part of the observed excursion in water level. A few days later the associated hydrograph on the 
Nueces reached Calallen and delivered a cumulative 3.3 Mm^ of inflow. Spread over the 500 km" of combined 
surface area of Corpus Christi and Nueces Bays, this would amount to less than a cm of additional water depth. 
Indeed, the water-level data from July show no discernible response to the Nueces inflow. 

The moral of this comparison is that hydrograph events on the river could be expected to have littie impact on 
the elevation of water in the bay, whereas sudden diluvial rainstorms may have, if the rainfall area encompasses 
a substantial portion of the bay area. Several such spikes can be seen in the water level histories that 
correspond to intense rainfalls. On the other hand, the Calallen flow hydrographs create a greater response in 
the salinity than a rainfall event. The former in fact is a water-mass displacement process, while the latter is a 
dilution of the rainfall depth throughout the water depth. 

The summer 1993 water-level history is a good example of the summer seasonal low water, due to the absence 
of other hydrographic factors from late June through August. In this same year, the subsequent fall high water 
is a rather minim al event. The 1 994 fall high water is more typical of this annual event 

The only significant hydrographic events occurring during the period after opening of the Nueces Overflow 
Channel but before the operation of the USGS Rincon gauge are foimd in October 1995, the month during 
which the channel was opened. A low but fairly steady flow over Calallen occurred during October but abated 
the day before the channel was opened. Iribeck observed that the level of water in the Nueces did not acquire 
the threshold to force flow through the overflow channel. On 28 October, two days after the channel was 
opened, an intense rainfall event (over 20 cm in one day) created sufficient local flow to scour down die 
channel (see Chapter 3 in the draft report). Although a spike ki bay water level occurs, the effect on salinity is 
negligible. For the next several months, only a few minor rainfall events appear in the record, and the bay 
salinity climbs, nearly monotonically, into the hyper-saUne range. 

Finally, it should be noted that there are other pathways for flow to enter Rincon marsh, vi2. the series of low 
points in the north levee of the Nueces River. Only when stage in the rivet becomes sufficiendy high does flow 
begin to pass these other openings. Based upon HEC-2 hydraulic model runs (Bureau of Reclamation 2000), a 
relation has been developed between the flow in the Rincon channel and the total flow entering Rincon marsh 
by aU of the available routes. The results of this modeling analysis are summarized in the following figure 
showing the proportion of total flow into the marsh represented by the Rincon channel. For small river stages, 
this is clearly 100% (Figure 2). Then the proportion rolls off in a sigmoid-like shape approaching a level of 
about 5%. Considering the sources of error in aU of this, the total flow into Rincon marsh can be 
approximately related to that in the Rincon channel as follows: 

Rincon Q Rincon Q < 200 cfs 

Combined Q (cfs) = 0.8 (Rincon Q - 200) Rincon Q 200 < Rincon Q < 450 cfs 

20 (Rincon Q) Rincon Q > 450 cfs 



Appendix B <* B-5 



# From HEC-2 model runs 

by US Bureau of Reclamation 




300 400 500 600 

Row (cfs) in Rincon channel 



Figure 2: Proportion of combined flow into upper Nueces delta carried by Rincon channel. Source of 
HEC-2 model run is Bureau of Reclamation (2000). 



INTERTIDAL (day to day) ANALYSIS 

The depiction of hydrographic time history in the Nueces delta on an intertidal time scale is based upon 
compiling the data sources and computing their integrated values for each day (24-hour period UCT) within the 
period of record. (The astronomical tidal period is actually 24.8 hrs, but an average over 24 hours eliminates 
almost all of the variability it contributes.) The term "integrated" means either averaged or accumulated, 
whichever is more meaningful for the parameter under consideration. The depiction is best presented either 
graphically or La tabular summary of the hydrographic variables. 

EVENT-DURATION ANALYSIS 

The intertidal (day to day) analysis covered the period of January 1992 through December 1999. After 
inspection of the entire period of record on both intertidal and intratidal scales, criteria were formulated to 
identify an "event" based upon the separate hydrographic behaviors of each of the key response parameters. 
These response variables included stage (in Nueces Bay, in Rincon Bayou and a super-elevation of the two), 
flow (in the Nueces River and in Rincon Bayou), and salinity (in Nueces Bay) (Table 2). It should be 
emphasized that these criteria were ultimately arbitrary, but were utilized to ensure an objective selection of 
candidate events for analysis. 

It is noteworthy that precipitation was not treated as a separate hydrographic variable, though it was certainly an 
important hydrographic element in imderstanding the response of the delta ecology. The reason for its 
exclusion as a defining parameter was that it provides no infotmMon per se on the response of the Nueces 
Estuary or Delta. A similar argument was made for excluding wind as a defining criterion. 

Once these criteria were established, the daily data for the period of study was manually inspected. Individual 
occiurences within the record which met at least one of the six criteria were identified as an "event". Then, for 
each event, the 24-hour mean data for all hydrographic variables during the event were separated and 
transferred for individual analysis. The duration period for each event was at least that for which the defining 



B-6 ♦ Hydrography of the Nueces Delta and Estuary: 1992-1999 



Table 2: Criteria used to define hydrographic events in the data record by response variables. 

Location Defining Criteria of a Hydrographic Event 



RESPONSE 
PARAMETER 



Flow 



Stage 



Salinity 



Nueces River A 24-hour nnean (daily) flow^ in the Nueces River at Calallen exceeding 

14.2 m^/s (500 cfs). 

Rincon Bayou A 24-hour mean (daily) flow In Rincon Bayou, either positive or negative, 

exceeding 0.28 m^/s (10 cfs). 

Nueces Bay A 24-hour mean (daily) stage in the vtrater elevation of Nueces Bay exceeding 

0.30 m (1.0 ft). [Referenced to the consistent CBI datum from Ward (1997), 
established by "empirical leveling".] 

Rincon Bayou A 24-hour mean (dally) stage in the w^ater elevation of Rincon Bayou 

exceeding 0.61 m (2.0 ft). [Relative to Rincon gauge datum, which is 422 cm 
above the consistent datum for CBI gauges.] 

Super-elevation The sum of Rincon Bayou minus Nueces Bay daily stage values exceeding 
0.15 m (0.5 ft). [Referenced to common datum] 

Nueces Bay Change in salinity concentrations of Nueces Bay exceeding 5 ppt over a five 

day period. 



criterion was satisfied, though often a longer event period was chosen to be sure that the complete response of 
the bay or delta was included in the analysis. When several hydrographic events overlapped {i.e., when several 
variables each satisfied criteria separately and simultaneously), the event duration was at least the period fi:om 
the first occurrence of the criterion threshold for the earliest parameter to at least the last such threshold for the 
latest parameter. 

The greatest difficulty in separating such events was met when a time series of events occurred in which the 
response of one parameter overlapped that of the next. For example, a series of river hydrographs might occur, 
each of which raises Rincon stage or Calallen flow above the threshold defining an event, and a new surge of 
inflow occurs before the recession of the preceding has subsided. Separating these into individual events was 
rather arbitrary, and from the estuarine response point of view, such a sequence might acceptably be considered 
one long event rather than a sequence of separate events. 

The parameters compiled for each event include the following: event number, date, duration, rainfall, flow, 
stage and salinity. 

Event Parameters 

Event Number 

For purposes of tracking and reference, each discrete event in the record was assigned an event label. Each 
event occurring from October 1, 1994, through December 31, 1999 (the duration of the demonstration 
project), was numbered sequentially in time, begirming with 1. Events occurring before the demonstration 
period were labeled with sequential letters, beginning with A. 

Date and Dutation 

The span of each event was specified by its starting date. In some cases, the ending date of one event was the 
starting day of the next, which indicates that a subjective separation had been assumed in the record for 
purposes of analysis. This arbitrary division may or may not have been capable of differentiation in the actual 
environment, which may have responded as if the two events were a single "merged" event. 

Rainfall 

For the period of October 1, 1994, through May 15, 1996 {i.e., prior to the activation of the Rincon gauge), local 
daily precipitation obtained fcom Corpus Christi airport, which evidenced a correlation with the USGS gauge 



Appendix B ♦ B-7 



on Rincon Bayou near Calallen (Station 0821 1503) (Rincon gauge) of 0.81 for the 3 years of coincident data. 
After May 15, 1996, daily precipitation was obtained by the Rincon gauge. 

Flow (Nueces River) 

Daily values for flow in the Nueces River were obtained from the U.S. Geological Survey (USGS) on the 
Nueces River at Calallen (Station 08211500) (Calallen gauge). Because no reliable field observations of 
discharge values were available above 77.87 m'/s (2,750 cfs), all daily flow values in excess of 77.87 m'/s were 
estimated by extrapolation (Irlbeck and Ockerman 2000). 

Flow (Rincon Bayou) 

For dates before the opening of the Nueces Overflow Channel (October 26, 1995), daily flow data into Rincon 
Bayou from the Nueces River were estimated from daily stage values recorded at the Calallen gauge using the 
method described by Irlbeck and Ward (2000). No daily data were available from the period of October 26, 
1995, through May 15, 1996, when the Rincon gauge was installed. From that date through the end of the 
record, flow data reported for Rincon Bayou was the total net daily flow gauged at the Rincon gauge. 

It is important to note that, during the demonstration period, the Nueces River did not exceed the natural 
flooding threshold for the delta, which is 1.71 m (5.60 ft) (Bureau of Reclamation 2000), except for on three 
occasions. This means that, except for these four events (Events 16, 18, 25 and 36), the only water exchanged 
between the Nueces River and Rincon Bayou passed through the Nueces Overflow Channel. During the three 
excepted events, an additional amount of water entered Rincon Bayou naturally via the low depressions along 
the bank of the river, and was estimated using the hydraulic model developed by Reclamation (2000). 

Stage 

Water level data for the Nueces Bay and Rincon Bayou were obtained from the Texas Coastal Ocean 
Observing Network (TCOON) marine monitoring system of Texas A&M University-Corpus Christi Conrad 
Blucher Institute (CBI) and the USGS Rincon gauge, respectively. The super-elevation of water levels, 
determined by subtracting the Nueces Bay stage from the Rincon Bayou value, was used to determine the 
predominant influence on stage in upper Rincon Bayou. On an instantaneous basis, the super-elevation is the 
direct force for discharge through the Nueces Overflow Channel. 

Salinity 

Salinity data were obtaitted from the CBI SALT03 gauge of the TCOON system. 

Observations 

Freshwater Flow 

A principal objective of this demonstration project was to increase the opportunity for partial diversion of flow 
events in the Nueces River through the Nueces Overflow Channel into Rincon Bayou and the upper delta. The 
project would thereby periodically increase water levels and inundate regions of the upper marsh, while at the 
same time reduce salinity concentrations, all of which were considered to be ecologically beneficial. Since the 
purpose of the demonstration project was to divert a portion of a flood hydrograph on the Nueces through the 
diversion channel, a logical inquiry was the proportion of such a flood so diverted. The bulk event data can be 
used to address this question. 

Upon examination of the relation between the total flow voliune in the Nueces River (for events which met the 
criteria) and in the Nueces Overflow Channel, it became obvious that there was a general association between 
the two. The volume diverted increased generally with the flow in the river, and the actual proportion of the 
flow amount diverted was on the order of 2 percent of that in the river (Figure 3). But this relation, such as it 
was, evidenced considerable scatter. In further analysis, the data was segregated by water level in Rincon Bayou 
at 0.3-m (1-ft) intervals. Within each class of water levels, the volume transported through the Nueces 
Overflow Channel proved to be substantially independent of the volume in the Nueces River. This was 
somewhat surprising. 



B-8 ♦ Hydrograph)/ of the Nueces Delta and Estuary: 1992-1999 



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Figure 3: Total flow volume carried in the Nueces River versus total flow volume diverted 

through the Nueces Overflow Channel. Only events that met criteria for a flow event in the 

Nueces River were used. The values plotted for Rincon flow volume are the total net exchange, or 

the integrated positive values. 

Note: 1 ft = 0.3046 m, 1 acre-ft = 1.2336 10' m' 

That the relation between Nueces River event volume and the volume transported through the Nueces 
Overflow Charmel should depend upon water level was not unexpected, based upon hydrauHc considerations. 
Unlike a river channel system in which the head gradient and the water level (stage) are closely related, there is 
no direct relation between water level and flow in the Nueces River below Calallen Diversion Dam because of 
the corrupting effect of tidal and meteorological water-level variations. The Nueces River hydraulic head is 
superposed on whatever water level is present in Nueces Bay. However, this water level does affect how the 
liver head can drive flow through the overflow charmel, because the deeper the water, the greater the cross- 
section area of the channel and upper delta, and the lower the frictional resistance. Therefore, a given hydraulic 
head in the Nueces River drives a greater flow through the diversion channel when die Nueces Bay water levels 
are higher. 

The surprising aspect of Figure 3 was the apparent constancy of the volume diverted versus river flow volume 
for a given class of water levels. For this there are two possible explications. The first is that this observation 
was an artifact due to die way that a hydrographic "event" was defined (which includes the entire period in 
which all of die variables respond, dien return to their pre-event values or, in die case of salinity, to a stable 
value). Thus the diuation over which Nueces River flow was computed is generally longer dian die duration of 
the flow event in the diversion channel. The rebuttal to this thought is that there is flow in the Nueces River 
that occurs when there is not flow in the overflow channel, so it was legitimate to integrate over die full 
hydrograph in die river channel. The second is diat this observation was a manifestation of die phenomenon 
of hydraulic capadt}', suggesting diat die Nueces Overflow Channel and upper Rincon Bayou very quickly reach 
their hydraulic capacity shordy after a flood event begins. The result is diat die volume diverted through the 
overflow channel becomes substantially constant, even as flow in the Nueces River increases. The present 
writers are inclined to this second view. If this constancy of volume is a valid inference, it would imply tiiat the 
2% proportion of flow in the Nueces River diverted into Rincon Bayou is itself an artifact of data points 
corresponding to different Nueces Bay water levels. 



Appendix^ ♦ B-9 



Water Level 

As with river flow, there was also a proportional iacrease in total flow through the Nueces Overflow Channel 
with an increase in super-elevation of stage. A s imil ar sorting by water levels also occurred when the 
event-duration data for flow volume in Rincon Bayou was plotted against the event mean super-elevation 
(Figure 4). In this figure, all 28 events were plotted, not just those events which met the criteria, as was done in 
Figure 3 Again it was observed that the larger flow volumes were associated with greater depths, independent 
of the magnitude of the super-elevation. Therefore, a given hydraulic head gradient drives a greater flow if the 
water is deeper. 



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Figure 4: Flow volume diverted through the Nueces Overflow Channel for all events versus 
event-mean super-elevation. 

Note: 1 ft = 0.3046 m, 1 acre-ft = 1.2336 10' m' 



Salinity 

Upon examination of the relation between salinity response of Nueces Bay before and after the opening of the 
Nueces Overflow Channel, all hydrographic events occurring from 1992 through 1999 were considered, rather 
than just those occurring after the initiation of the demonstration project. The incremental percentage salinity 
responses (%chg) of each event was plotted against the total event flow volume in the Nueces River (Figure 5). 
Data for events during which the starting salinity was less than 5 ppt were not included due to "noise" in the 
salinity measurements themselves. Several inferences immediately followed an inspection of this figure. 

First, for total event flows greater than 12,336 10^ m^ (10,000 acre-ft), there was no obvious difference between 
the fractional salinity response of events occurring before the opening of the overflow chaimel and those after 
it. That is, the same general relation of diminishing percent salinity response with increasing event flow in the 



B-10 ♦ Hydrography of the Nueces Delta and Estuary: 1992-1999 



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Figure 5: Fractional salinity response versus flow volume in Nueces River (measured at Calallen). 

Note: 1 acre-tt= 1.2336x10' I 



Nueces River was observed within the scatter of the data. That water diverted through the overflow channel 
would not measurably affect the response of bay saUnity was not surprising, given the very small proportion of 
flow volume diverted through the overflow channel compared with that in the river. 

Second, the rate of decrease of the fractional response (that is, the increase of negative values) with increasing 
flow in the Nueces River appeared to asymptotically trend to -100%. Considering that the mean water level 
volume of Nueces Bay is about 49,344 10' m' (40,000 acre-ft) (Ward 1997), one might have expected the 
fractional responses to equal -100 percent for flow volumes exceeding this 12,336 10' m'. But this was not the 
case, probably because the definition of an "event" may encompass some of the period of salimty recovery. 
Moreover, there were also hydrographic processes that enabled water from lower in the bay (and containing 
higher salinity) to infiltrate back into Nueces Bay during the event period, including tides and wind forcing. 
Finally, the adjective effect of the Nueces River depended not only upon its volume, but also the period over 
which this volume is delivered. 

Finally, for total event flow volumes in the Nueces River of less than 12,336 10' m' (10,000 acre-ft), the 
fractional salinity response became extremely noisy, and even became positive for a significant number of the 
events. Restating this observation in another way, below a threshold volume in the river, other non- 
hydrological factors become equally or more important in affecting the salinity of Nueces Bay, and the apparent 
value of this threshold is around 12,336 10' m'. Similar threshold-type controls on salinity were theoretically 
expected and have been found to operate when salinity and flow data are adequate to characterize the response, 
such as in Trinity Bay in the Galveston system. Although quantification of this threshold was not relevant to 
evaluating the effects of the subject demonstration project, it may be important in devising operating strategies 
for future such diversions. 



Appendix B ♦ B-11 



LITERATURE CITED 

Bureau of Reclamation. 2000. Hydraulic i\nalysis of Rincon Bayou Using HEC-2, Rincon Bayou-Nueces 

Marsh Wetlands Restoration and Enhancement Project. United States Department of the Interior, Bureau 
of Reclamation, Great Plains Regional Office, Billings, Montana. 

Irlbeck, M.}. and D. Ockerman. 2000. Technical notes on the Rincon gauge and data. Unpublished. 

In Concluding Report: Rincon Bayou Demonstration Project, Appendix I-A. United States Department of 
the Interior, Bureau of Reclamation, Austin, Texas. 

Idbeck, MJ. and G.H. Ward. 2000. Analysis of the historic flow regime of the Nueces River into the upper 
Nueces Delta, and of the potential restoration value of the Rincon Bayou Demonstration Project. 
UnpubUshed. /« Concluding Report: Rincon Bayou Demonstration Project, Appendix I-B. United States 
Department of the Interior, Bureau of Reclamation, Austin, Texas. 

Ward, G.H. 1997. Processes and trends of circulation ^vithin the Corpus Christi Bay National Estuary Program 
Study Area. Report CCBNEP-21, Corpus Christi Bay National Estuary Program, Corpus Christi, Texas. 



B-12 ♦ Hydm^aphy of the Nueces De/ta and Estuary: 1992-1999 



APPENDIX C 



Analysis of the Historic Flow Regime 
of the Nueces River into the Upper 
Nueces Delta and of the Potential 
Restoration Value of the Rincon Bayou 
Demonstration Project 



Michael J. Irlbeck U.S. Bixreau of Reclamation, Austin 

Dt. George H. Ward Center for Research in Water Resources, University of Texas, Austin 

In preparation for publication. 

INTRODUCTION 

Deltaic ecosystems are typically supported by at least one principal river system, and therefore have adapted to 
and are dependant upon the dynamic fluctuations of that river's flow pattern. This natural flow regime includes 
the fuU range of a river's flow quantity, timing and variability, which may fluctuate by hours, days, seasons, years 
and even decades. Until recendy, the importance of the natural stream flow variabUity in maintaining healthy 
aquatic ecosystems has been underappreciated in a water development and management framework (Kart 
1991, Voiiet al. 1997). However, it has been shown that the integrity of flowing water systems depends largely 
upon the components of their natural dynamic character, which is critical in regulating a variety of ecological 
processes within these ecosystems (Poff and Ward 1989, Richter et al. 1996, Walker et al. 1995). 

In south Texas, the ecological integrity of the Nueces Delta has been primarily determined by the natural flow 
regime of the Nueces River, which includes the magnitude, frequency, duration and timing of flow events in the river. 
Occasionally, high flows in the Nueces River cause the water elevation to exceed the diversion threshold (or, 
rise above the lowest point along the river bank) and spill into the upper delta. These periodic freshwater 
inundation events drive several key biological processes important for estuary productivity (Longley 1994). The 
magnitude of a given flow event is simply the amount of water moving past a fixed location per uiut time. 
Event duration is the period of time associated with a specific flow condition. The frequency of an event refers 
to how often a flow at a given magnitude recurs over some specified period of time. And the timing of events, 
or their seasonal predictabiUty, refers to the regularity with which flow events of a specified magnitude occur 
within a specified time scale. 

In 1993, as part of a broader initiative to restore freshwater flows to the greater Nueces Estuar)', the United 
States Bureau of Reclamation (Reclamation) began a multi-year demonstration project designed to increase the 
opportunity for natural fireshwater flow events to enter the upper Nueces Delta. A key component of the 
Rincon Bayou Demonstration Project was a 900-m long overflow channel which was excavated to coimect the 
Nueces River with the extreme upper reach of Rincon Bayou, the dominant hydraulic feature of the upper 
delta. A second overflow channel, which connected Rincon Bayou with a broad area of barren tidal flats in the 
upper delta, was also excavated. These features effectively lowered the flooding threshold of the upper delta 
and improved the distribution of diverted freshwater. 

If the historic flow regime characteristics of the upper Nueces Delta could be defined for the recent past, the 
impacts of human activities, particularly that of reservoir development and management within the Nueces 
River watershed, could be analyzed expUddy. These same components could also be used to evaluate the 
potential alterations to the natural flow regime of the delta system resulting from a variety of restoration or 
enhancement activities. 

Appendix C ♦ C-1 



OBJECTIVES 

The objectives of this analysis were to characterize, through analysis of the magnitude, frequency, duration and 
timing of flow events: 

1) changes in the historic freshwater flow regime of the upper Nueces Delta, and 

2) the potential for restoration of freshwater flow resulting from the Rincon Bayou Demonstration Project. 

BACKGROUND 

Description of the Study Area 

As the Nueces River flows toward Corpus Christi Bay along the south Texas coast, it passes along the southern 
edge of a large delta located in southern San Patricio County (Figvue 1). This delta is an integral part of the 
Nueces Estuar)', and is roughly 70 square kilometers (km) in size. The water surface for most of the channels 
and ponds in the delta is very near sea level, and the low-l^ing flats are intermittendy inundated from the bay by 
tides and storm surges. The delta is crossed from north to south by numerous buried pipelines and two 
Missouri-Pacific railroads. One of the most dominant hydraulic features of the upper delta is a broad, tidally- 
influenced channel known as Rincon Bayou. Between the railroads, the upper delta is separated by a modest 
ridge of high ground along the southern bank of Rincon Bayou. There is some evidence that this crest is what 
remains of early diking efforts in the delta, probably for agricultural purposes (US Ejigineer Office 1939). 
Downstream of the eastern-most railroad, the elevations are more uniform and tidal interactions make a 
separation of the northern and southern delta less distinct. 

The Nueces River Basin has a total watershed area of approximately 44,224 km^. Occasionally, larger flood 
flows in the river spill over the northern bank and inundate the delta with freshwater. These events usually 
occur shortiy after periods of heavy rainfall in the basin, usually coinciding with tropical storm activit)' in early 
fall or with the passage of frontal systems in late spring. These sporadic flooding events supply freshwater to 
plant communities, transport detrital materials from the vegetation and sediments to the bay, provide a medium 
for nutrient exchange and buffer bay salinity. The Nueces Delta marsh is therefore one of the most important 
sources of nutrient material for the Nueces Estuary system (Texas Department of Water Resources 1981). 

Historical Changes in the Study Area 

Physical Changes in the Nueces Delta 

From a broad perspective, large-scale physical form of the delta has changed Utde during the period under 
review {i.e., 1940-1999). Through comparisons of current and historic maps, each of the two railroads which 
cross the delta from north to south were in places by 1940, and most of the larger pools and channels in the 
delta resembled their present locations, shapes and sizes (US Engineer Office 1939). The two railroad crossings 
were elevated by fill material for most of their span, with the exception of a few bridged crossmgs over the 
more significant channels. This construction method undoubtedly changed the fundamental hydraulics of the 
delta system by isolating, restricting, and channelizing some it's water courses, but these changes had already 
occurred well before the beginning of the period under consideration {i.e., 1940). 

The road bridge over the Nueces River near Calallen has been in place since before 1930, but has been rebuilt 
on several occasions (Texas State Highway Department 1931). In 1931, a two-lane tresde bridge was 
constructed by the State of Texas as part of improvements to what was then called State Highway 9. This 
structure was removed and replaced with a standard bridge in 1956, and then improved to a four-lane bridge in 
1959 as part of the upgrade to Interstate Highway 37 flexas State Highway Department 1959). Finally, in 1983, 
the Interstate Highway 37 (IH 37) bridge was upgraded to its current form. 

Presendy, the flooding threshold for the majority of the north bank of the Nueces River in the upper reaches of 
the delta is about 2.36 m (7.75 fit) msl, although some lower channels are present. Given the natural effects of 
scouring and deposition during large flow events, the Nueces River has likely cut and filled a coundess number 
of depressions in its geologic past, continually altering the flooding threshold with each event. In addition to 
natural causes, human activity has also contributed to this process of change. As one example, numerous large 
pieces of concrete rubble and re-bar have been unearthed along the north bank of the river just downstream of 

C-2 ^ Analysis of the Historic Flow Reffme of the Nueces River into the Upper Nueces Delta 



Approximate Scale 




Figure 1 : The Nueces Delta. Generally depicted are open water (shaded) and tidally influenced areas. 
Also shown are the overflow channels that were part of the Rincon Bayou Demonstration Project. Sources 
of base map: Salas 1993. 

IH 37 (Bureau of Reclamation 2000). It is speculated that this material was intentionally placed in low portions 
of the bank to reduce flooding of adjacent pastures, and was probably acquired from the road bridge renovation 
during the late 1950's. 

Reservoir Consttuction in the Nueces Watershed 

Reservoir development in the Nueces Basin has primarily been initiated by the population centers along the 
coast in efforts to secure a reliable freshwater supply. The fundamental objective of building such 
impoundments was to capture large flood events in reservoirs instead of allowing the water to pass into the 
bays. Stored water could then be released slowly for municipal and industrial consumption, thereby providing a 
dependable freshwater supply for the community, especially during the periods of frequent drought. 

The largest of these communities is the City of Corpus Christi, which was incorporated in 1852. At that time, 
its citizens obtained their domestic water from shallow wells, cisterns, and natural depressions existing in an 
arroyo that ran through town. However, during drought conditions, the City often suffered from water 
shortages when the cisterns and tanks became low or dry from lack of rainfall. Bay water was used to meet any 
fire fighting needs. In 1887, a committee appointed by the City investigated the possibihty of obtaining an 
adequate water supply from local ground water, but exploration drilling found the source to be brackish and 
unsuitable for domestic purposes. 

Abandoning the idea of using local groundwater, the City looked to the Nueces River. In 1893, the City 
finished construction of a steam-powered pumping station and distribution system that would provide raw 
water to the City. The pumping station was located on the south bank of the Nueces River at Calallen, some 16 
miles to the northwest of the City. To prevent saltwater from being pumped and distributed during low- flow 
periods, the City constructed a temporary wooden dam (Calallen Dam) on the Nueces River just downstream 



Appendix C ^ C-3 



of the intake. Then in 1898, a pennanent rock-filled dam with a crest elevation of 0.46 m (1.5 ft) msl was 
completed at the same location. In 1916, the first treatment plant was added at the Calallen pump station. In 
1935, the current concrete structure known as Calallen Diversion Dam was completed with a crest elevation of 
approximately 1.36 m (4.52 ft) msl. 

During 1917, another drought threatened the municipal water supply of the growing City of Corpus Christi and 
the surrounding area. A series of engineering surveys identified a suitable site for a large reservoir on the 
Nueces River some 35 miles upstream of Calallen Dam. The purpose of the new dam would be to store water 
for release during the dry periods. Mathis Dam was originally constructed by the Cit}' of Corpus Christi early in 
1930, but failed in November of that same year. In 1934, a second dam across the Nueces River (La Fruta 
Dam) was constructed. This new reservoir had a storage capacity of approximately 67,849 10"* m' 
(55,000 acre-ft). 

From 1930 to 1950, the population of the Corpus Christi area increased by over 400% (Corpus Christi 1990). 
As a result of this increase in demand for municipal and industrial water, and from the loss of reservoir storage 
in La Fruta Reservoir due to siltation, a new water source was sought, again from the Nueces River. After 
several years of study, the State of Texas financed the design and construction of Wesley Seale Dam (Lake 
Corpus Christi), which was completed in 1958. This dam was located about 300 m downstream from La Fruta 
dam site and was some 6 m higher, inundating the former structure. During the first six years of operation. 
Lake Corpus Christi was maintained at 26.8 m (88.0 ft) msl, or a capacity of 229,355 10^ m' (185,922 acre-ft), to 
allow for depletion of oil fields located in the reservoir basin. The crest gates were finally closed on July 1, 
1964, bringing the operational lake elevation to 28.6 m (94.0 ft) msl, thereby increasing the storage capacity to 
372,550 10' m' (302,000 acre-ft). 

Forecasts of future water requirements for the Coastal Bend area indicated that demand would exceed the firm 
aimual yield of Lake Corpus Christi during the 1 980's. In response to the Area Development Water 
Subcommittee's recommendation, the City of Corpus Christi engaged the Bureau of Reclamation to sur\'ey the 
lower Nueces River Basin to determine a feasible location for a new reservoir to supplement Lake Corpus 
Christi. Reclamation began construction on Choke Canyon Dam in the summer of 1979, and the project was 
declared "substantially complete" on May 18, 1982. Due to a drought affecting the Frio River's 14,323 km" 
watershed, flow into the new reservoir was minimal during the first three years, and by May 31, 1987, the 
reservoir was only at 48% of capacity. However, record rainfall on the Frio River watershed filled the reservoir 
to 100% on June 18, 1987, and water was released as flood control for the first time. At this level. Choke 
Canyon Dam impounds approximately 852,584 10' m' (691,130 acre-ft). 

Developed for the purposes of providing a reliable and municipal water supply and flood protection, these 
dams have contributed to reduced streamflow in the lower Nueces River by their diminutive influence on larger 
river hydrographs, and through direct water loss to consumptive uses and evaporation. The present permitted 
firm yield of the reservoir system is 139,000 acre-ft, and a portion of the dehvered water returns to the estuary 
through treated return flows. Because of the relative shallow depth of the two reser^'oirs and the hot summer 
climate, evaporation from these two water bodies can remove a significant amount of water from the river 
system. For example, during 1999 alone, over 217,730 10' m' (176,500 acre-ft) were lost to evaporation from 
the combined reservoir system (Hilzinger 2000). 

Other Changes in the Nueces Watershed 

Another possible factor contributing to decreased stream flow in the lower Nueces River are increased non- 
reservoir surface water withdrawals in the greater watershed. For example, long-term (1940 to 1990) analysis of 
reported surface water withdrawls m the basin upstream of the reservoirs indicates an increase of about 60% 
from 1965 to 1990 (Greene and Slade 1995), which includes much of the operational time period of the current 
reservoir system. 

Also, a decreasing precipitation trend in the Nueces River watershed would be expected to reduce streamflow. 
However, after analyzing rainfall data from four south Texas gauges (Cotulla, BeeviUe, Sabinal and Corpus 
Christi) reflecting conditions for the Nueces watershed, Medina (2000) found that annual precipitation (using a 
base period that consisted of data since 1900) produced no particular trend (Figures 2a through 2c). Using a 
baseline that began during the late 1940's (e.g.. Figure 2d), annual precipitation portrayed an increasing trend 
(Medina 2000). The most prominent and common feature of the precipitation data at all stations was the 
drought of the late 1940's and early 1950's. Similarly, Asquith et a/. (1997) also found Utde evidence for 
statistical trends in precipitation along the Coastal Bend from 1968 through 1993. 

C-4 ^ Analysis of the Historic Flow B^ffme of the Nueces River into the Upper Nueces De/ta 



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10 




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^oP\c,NV\<*'^\^'^V\<^*\<^'^\<**''N<i'*V'' 




Approximate Scale 

Kilomelers 



Figure 2: Annual precipitation trends at four gauges about the greater Nueces River watershed 
since about 1900. 



Source: Medina 2000. 



Note: 1 inch = 2.54 cm 



Appendix C ♦ C-5 



METHODS 

As previously discussed, the Nueces Delta has been a dynamic system in the past, experiencing changes from 
both natural and man-made causes. During the past 60 years, the major attributes of the delta Hke general 
topography, railroad crossings and channel locations have not changed significandy, while smaller 
modifications, such as flooding thresholds, have occurred. However, lacking the information necessary to 
define more subde changes in the study area or to analyze their effects, the assumption was made for the 
purposes of this analysis that the delta's physical characteristics during the entire period under study have 
essentially resembled those observed in the field during 1993. 

Data Sources 

Available data included daily discharge and stage 
records for several gaizges on the Nueces River 
operated and maintained by the United States 
Geological Survey (USGS) (Figure 3). 

Mathis Gauge 

Station 08211 000, the Nueces River Near Mathis, 
Texas (Mathis gauge), is located on the Nueces River 
0.96 km downstream of Wesley Seale Dam (Lake 
Corpus Christi). The gauge receives flow from 
approximately 43,512 km^ or 98.4%, of the 
watershed. Daily discharge and stage data were 
available from August 1 939, to present. The 
maximum daily stage value recorded at the Mathis 
gauge was approximately 3,546 cubic meters per 
second (mVs) (125,000 cfs) on September 25, 1967. 

Calallen Gauge 

Station 08211500, the Nueces River at Calallen, 

Texas (Calallen gauge), is located on the Nueces 

River 0.64 km upstream from Calallen Diversion 

Dam, approximately 2.3 km upstream from the 

bridge on Interstate Highway 37 and some 54.6 km 

downstream from the Mathis gauge. This gauge 

receives flow from approximately 44,071 km^, or 99.7% of the watershed. Unpublished daily stage data were 

available for the period of April 1920 through July 1950 from the USGS District office in Austin, Texas. 

Reliable daily discharge and stage data are published from October 1989, to present. The Calallen gauge is 

operated as a low-flow gauge, with daily discharges published only for days when instantaneous maximum 

discharge does not exceed 72.8 m^/s (2,570 cfs) (Gandara et al. 1996), which corresponds to a daily stage value 

of about 2.48 m (8.14 ft). However, higher daily stage values up to 4.13 m (13.55 ft) were available from the 

unpublished record. Datum of the gauge is 0.84 ft above mean sea level. 

Rincon Gauge 

Station 0821 1503, Rincon Bayou Channel near Calallen, Texas, (Rincon gauge) was installed and operated by 
the USGS at the request of the U.S. Bureau of Reclamation as part of their Rincon Bayou Demonstration 
Project. The objective of this project was to increase the opportunity for freshwater flow events into the upper 
Nueces Delta. The main feature of the demonstration project was a 305-meter (m) overflow channel excavated 
from the Nueces River at the point of natural diversion to a small headwater of Rincon Bayou. The Rincon 
gauge is located in this headwater channel approximately 310 m downstream from the north bank of the 
Nueces River. Daily stage, discharge and precipitation data are available from May 1 996 through December 
1999. The maximum daily stage value recorded at the Rincon gauge was 2.21 m (7.25 ft) on October 21, 1998. 
Datum of the gauge is at mean sea level. 



^^^B SAN PATRICIO 




Approiimale Scale 
Kilomalafi 


Corpus Christi 




12 3 4 5 


LIVE OAK Wesley Seale Dam 






MATHIS 1 Nueces 
Gauge f R'^^r 














^ Hondo Creek 


JIM WELLS V*i»<\~~~--. 


-^ 


~\ RINCON 
j Gaugfr 


NUECES Calallen Div(| 


gion 


( fA Upper 

^V^ Nueces 

Dam^ r^. 

calallenV* 

Gauge 



Figure 3: Diagram of the lower Nueces River 
showing the location of selected stream flow 
gauging stations.. 



C-6 V Analysis of the Historic Flow Reffme of the Nueces River into the Upper Nueces Delta 



Developing a Daily Stage Record for the Nueces River at the Point of Diversion 

The analysis of the Nueces River's flow regime and associated delta flooding characteristics over the past 60 
years required a daily stage record for the Nueces River at the point of natural diversion, which was not 
available. The "point of natural diversion", for the purposes of this investigation, was generaUy defined as the 
2,000-m reach of the north bank of the Nueces River from Interstate Highway 37 downstream to where it 
sharply bends to the south (Figure 4). In lieu of actual gauge data for this reach of the river, an artificial stage 
record was developed from correlations of data collected at other gauges in the lower Nueces watershed. This 
simulated daily record was then used to generally represent flow conditions at the point of diversion. 




Figure 4: The "point of diversion" along the north banl( of the Nueces River. The location of the five 
natural depressions in the river bank which contribute to delta inflow during a flood event are indicated by 
arrows (Bureau of Reclamation 2000). 

Although measures were taken to minimize the error Ln the manufactured data record, the accuracy of specific 
quantitative values estimated by this method is debatable. However, this method does provide a reasonable 
estimate of general hydraulic conditions in the Nueces River and upper delta during the period under review, 
and therefore was considered to be acceptable for use in analyzing qualitative {i.e., relative) differences in flow 
event characteristics. Furthermore, this analysis was exclusively focused on the upper Nueces Delta, or that 
northern portion of the delta primarily influenced by Rincon Bayou. No attempt was made to characterize the 
flow regime of the lower (or southern) portions of the delta. 

The procedure used to create a daily record of discharge into the upper Nueces Delta from daily Mathis 
discharge data included two basic steps. First, gauge data was used to correct and correlate Mathis discharge 
with stage values at Calallen (the Mathis gauge provided the longest reliable daily flow and stage data for the 
longest continuous period of record on the lower Nueces watershed, or from August 1939 to present). Second, 
gauge data was used to correlate daily Calallen stage with stage values at the point of diversion. 



Appendix C ♦ C-7 



Correlating Mathis Discharge to Estimated Stage at Calallen 

Data from the concurrent periods shared by the Mathis and Calallen gauges (daily records from January 1940 
through July 1950, and October 1989 dirough December 1999) were used to correlate Mathis discharge to Caktikn 
stage. Prior to this correlation, several modifications to the original Mathis data set were made. 

First, the travel time of flow events was examined for the period of conctirrent data between the Mathis and 
Calallen gauges. Based upon this analysis, the average travel time for flow events in the Nueces River to travel 
from Mathis to Calallen was determined to be about two days. The entire Mathis data set was therefore 
corrected by delaying each daily flow value by 48 hours. When analyzed individually, however, the actual travel 
time for the flow peak of any one flow event, which depends greatly upon the characteastics of the individual 
flood event, has been estimated to take as many as 5 days (Ward 1985). The average correction was used 
because individual correction of each flow event was not possible for the entire data set due to the absence of 
daily stage data at Calallen for the period of 1950 through 1989. 

Second, the resulting data set was corrected to account for channel losses in the reach between the Mathis and 
CalaUen gauges. HDR Engineering, Inc., et al. (1991) calculated an average loss rate of 0.1243% per river 
kilometers, or a total of 7.0% for the entire 56.3 km reach between Lake Corpus Christi and Calallen Dam. 
This rate was based on field measurements reported by the United States Geological Survey (1968), and is 
representative of the loss rate during periods of normal water deliveries with minimal intervening flows. The 
distance between the Mathis and Calallen gauges is only 54.6 km, sUghdy shorter than between the two dams, 
which resulted in only a 6.8% loss rate. A conservative arbitrary value of 8.5 m'/s (300 cfs) was used to 
represent the upper limit of " minim al intervening flow". Daily Mathis values below this limit were corrected by 
a loss of 6.8%. Stream flow losses for daily values in excess of 8.5 m'/s were assumed to be constant at 
0.58 mVs (20.4 cfs), or 6.8% times 8.5 mVs. 

Third, the resulting data set was corrected to account for the estimated total daily municipal and industrial 
withdrawals made from the Nueces River at or before Calallen Diversion Dam. Estimated total annual 
withdrawals for every tenth year of the period of record were derived from a number of sources (Homer & 
Shifrin Consulting Engineers 1951, Bureau of Reclamation 1971, Corpus Christi 1990), and then estimated for 
each intervening year assuming a linear relationship between the decade totals. Each total annual withdrawal 
amount was then divided by an average monthly percentage of water use for the Coastal Bend area (Bureau of 
Reclamation 1971), and the resulting values converted from total volume to average daily flow. These daily 
values, which represent the estimated daily municipal and industrial withdrawal for that month and year, were 
then subtracted from the modified Mathis data set. The daily correction values range from 0.20 to 0.25 m'/s 
(7 to 9 cfs) in 1940, and from 4.36 to 6.20 mVs (154 to 219 cfs) in 1999. 

Once the Mathis data set had been thus corrected, the relation between this discharge data and published 
CalaUen stage data was determined through linear regression (Figure 5). Daily values greater than 2.44 m (8.0 ft) 
from the unpublished Calallen data were also used. In addition, one point was added to the data set as an 
estimate of the extreme condition. The largest daily flow value recorded at Mathis was 3,539 m'/s (125,000 cfs) 
on September 25, 1967, and was a result of the massive flooding caused by Hurricane Beulah. This event 
occurred during the period for which there is no available stage data at Calallen, but other sources have 
reported that the Nueces River at Calallen crested at 5.02 m (16.48 ft) the same day (Corpus Christi Times 
1967). 

This relationship therefore allows the use of Mathis daily discharge data as the independent variable to solve for the corresponding 
estimated daily stage at Calallen. The entire corrected Mathis discharge data set was then converted to estimated 
stage at Calallen using this relation. 

There are three primary limitations to this method of correlating Mathis and Calallen gauge data. First, as 
previously discussed, travel times for each flow event vary, which compromises the temporal accuracy of the 
estimations. The inability to correct each flow event in the record for travel time requires the acceptance of this 
error. Second, although there is very litde additional watershed below Mathis and above Calallen (only 559 
km'^, some locally intense storms produce flow events at Calallen that are not recorded at Mathis. This 
relational difference between the Mathis and Calallen gauge data sets is especially high for tropical storm events 
that move onshore from the Gulf. Finally, stage data for the Nueces River at Calallen is not available for values 
in excess of 4.13 m (13.55 ft) (which roughly correspond to a discharge of about 50,000 cfs at Mathis), requiring 
extrapolation for higher discharge values. 



C-8 ^* Analysis of the Historic Flow Reffme of the Nueces River into the Upper Nueces Delta 



c 

CO 

O 

ro 

<D 
05 
TO 

CD 

Q 



17 

16 

15 

14 

13 

12 

11 

10 

9 

8 

7 

6 

5 

4 

3 



I I ^* 

::=^^» :: -' - 



20000 40000 60000 80000 100000 

Corrected daily discharge at Mathis (cfs) 



120000 



Figure 5: Regression analysis of corrected daily Mathis discharge and daily stage at Calallen. 

Note; 1 (rfs = 0.0283 m'/s; 1 ft = 0.3046 m 

Correlating Calallen Stage to Estimated Stage at the Point of Diversion 

Stage or flow data for the Nueces River downstream of Calallen Dam is extremely limited, but were available 
from the Rincon gauge for a period of approximately i'A years. This gauge was located in the headwater 
channel of Rincon Bayou, approximately 310 m downstream from the point of natural diversion of Nueces 
River. Therefore, the concurrent period shared by the Calallen and Rincon gauges (daily flow and stage records 
from May 1996 through December 1999) was used to correlate Calallen stage to estimated stage at the point of 
diversion. This included one modification to the original Rincon data set prior to the correlation. 

At higher stage values, the Rincon gauge experienced a minor head loss during discharge events, which was 
estimated to be as much as 0.08 m (0.25 ft) by the Bureau of Reclamation using a hydraulic model (2000). The 
Rincon stage data was therefore corrected using this model to represent stage at the point of natural diversion 
before being correlated with Calallen gauge data. 

Once the Rincon data set had been corrected, the relationship between daily Calallen stage values and corrected 
daily stage data at the point of diversion was determined through linear regression (Figure 6). Not used were 
data from several anomalous stage events recorded at the Rincon gauge (October 1996, November 1996, April 
1997, June 1998, and May 1999) which were not associated with discharge at Calallen. In addition, data from 
two previous maximum stage events were used as estimates of the extreme conditions. First, during the flood 
of 1919, the maximum stage of the Nueces River recorded at Calallen by the USGS was 4.16 m (13.65 ft), and 
at the Interstate Highway bridge (representing point of diversion) was 3.75 m (12.3 ft) (Texas State Highway 
Department 1956). Second, during the 1967 flood, the maximum stage recorded at Calallen was 5.02 m (16.48 
ft) (Corpus Christi Times 1967), and at the bridge was 4.63 m (15.2 ft) ^exas State Department of Highways 
and Public Transportation 1983). 

This relationship therefore allows the use of Calallen daily stage data as the independent variable to solve for the corresponding 
estimated daily stage at the point of natural diversion. The entire estimated Calallen stage data set was then converted 
to estimated stage at the point of diversion using this relation. 



Appendix C ♦ C-9 



15 - 
"^ 14 - 
^ 13 




























^ 


























































" 12 - 






















♦ 








1 11- 

^ 10 - 

a 9 - 

o 8 


















































































































-I-' 

•i 7- 

i 6- 
5 - 
CO 4 - 






































♦ ^ 


► 


























# 
























♦ 


ll 


r 






















A 


if 


iP 


r 




















Daily 


m^ 


w 






















i 


^ 


P 
























n . 





























3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 

Daily stage at Calallen (ft) 



Figure 6: Regression analysis of Calallen daily stage values and daily stage at the Point of 
Diversion. 



Note: 1 ft = 0.3046 m 



There are three primary limitations to correlating Calallen stage to stage at the point of diversion. First, under 

certain conditions, Calallen gauge data does not represent the total river discharge under the IH 37 and past the 

point of diversion. At higher flows, a portion of the Nueces River spills out of the stream channel upstream of 

the gauge, and re-enters the river via Hondo Creek below Calallen Dam but above the point of natural 

diversion. This portion of the river's flow, 

although relatively small, evades detection 

at the Calallen gauge (Figures 3 and 7). 

Second, a variety of tides, storm surges 

and other hydrographic events in Nueces 

and Corpus Christi bays also affect the 

Rincon stage data. Because the Calallen 

gauge is insulated from all but the most 

extreme of these events by Calallen 

Diversion Dam, correlation between the 

two data sets, especially under low-flow 

conditions, was "noisy". Finally, the lack 

of a long-term record at Rincon limits the 

ability to analyze a full range of hydraulic 

conditions in the Nueces River. 

In summary, the first two steps of this 
methodology produce a daily stage record 

which represents the approximate water ^'^^'^ ^r View of Hondo Creek under flood conditions 

level of the Nueces River at the point of ^""f 26. 1 997. This road crossing is located approximately 

... r ^ J f 2 miles upstream of the creek s confluence With the Nueces 

natural diversion for the penod of ^^^^ ^^^^^ .^ downstream of the Calallen gauge. The flow is 

January 1, 1940, to December 31, 1999. 3 ^^^^^ ^^ ^^^^^ ^^^^ ^^^ ^^^^^ pj^g^ at a point further 

upstream. Ptioto courtesy of the Bureau of Reclamation. 




C- 1 ^ Analysis of the Historic Flow Rtffme of the Nueces River into the Upper Nueces Delta 



Estimating Freshwater Inflow into the Upper Nueces Delta 

Once a representative daily stage record had been developed for the Nueces River at the point of diversion, two 
different sets of stage-discharge rating curves were used to estimate daily discharge into the upper delta. Prior 
to this analysis, the Bureau of Reclamation, as part of their Rincon Bayou Demonstration Project, conducted 
(1996) and revised (2000) a hydraulic study of the relation between flow in the Nueces River and that in Rincon 
Bayou. These modeling efforts, which did not address the southern portion of the delta, produced a series of 
rating curves based on field conditions during March of 1993. Among the scenarios developed were the pre- 
project (or historical) condition, and the post-project (restored) condition. 

Historical Freshwater Inflow into the Upper Nueces Delta 

Under the without-project (historical) scenario analyzed by the Bureau of Reclamation (2000), a total of five 
natural depressions in the north bank of the Nueces River were identified tliat would naturally contribute to 
discharge into the delta at various stages in the river (Figure 4). The lowest of these drainage channels was 
along the west side of the Missouri-Pacific railroad bridge, which had an effective bottom elevation of about 
1 .64 m (5.4 ft) msl. The hydraulic characteristics of each of these depressions were combined into one 
commutative set of rating curves, including both a rising and falling limbs, for daily discharge into the upper 
Nueces Delta (Figures 8 and 9). The obvious 'Tsend" in the rating curves reveal the natural flooding threshold 
for the greatest part of the river bank, which is about 2.36 m (7.75 ft) msl. Reclamation's without-project rating 
curves were used without modification, and estimated daily stage values for the Nueces River at the point of 
diversion were converted to estimate daily discharge into the upper Nueces Delta. 

Potential for Restored Freshwater Inflow into the Upper Nueces Delta 

Base on data obtained from the Rincon gauge, the Bureau of Reclamation (2000) also constructed a set of rating 
curves, including both rising and fallin g Umbs, which estimated daily discharge into the upper Nueces Delta 
with the demonstration project features in place (restored condition) (Figures 8 and 9). Because of the 
compromising effect of tide on water elevations at the point of diversion, and therefore on discharge estimates, 
these curves did not estimate discharge into the upper delta when the stage in the Nueces River was 0.76 m 
(2.50 ft) or below. 

However, when discharge estimates for individual events using the falling limb curve (which represented an 
average of several observed events) were compared with actual discharge data from the Rincon gauge, the 
results were unsatisfactorily inconsistent. Upon examination, it was discovered that, although the falling limb 
curve of each event expressed the same slope when plotted, the beginning point of each curve depended upon 
the maximum water surface elevation attained by the Nueces River during that particular event. Therefore, a 
series of fallin g Umb curves were subsequendy constructed by extrapolation of the average curve in 0.1 -ft 
intervals. Discharge estimates for individual events using the modified, event-specific set of rating curves were 
then tested for accuracy, this time with acceptable results. For each of the eight (8) major freshwater flow 
events recorded at the Rincon gauge, the estimated discharge was within about 10% of the actual for five 
(5) events, and within about 25% of the actual for two (2) others (Table 1). The combined accuracy for all 
discharge estimates made using the revised rating curves for the restored condition, including all eight events, 
was about 14% over the actual gauged discharge value. 

Once an acceptable set of rating curves for the "restored" condition was thus developed, the estimated daily 
stage values for the Nueces River at the point of diversion were converted to estimated daily discharge into the 
upper Nueces Delta. 

Definition of Flow Regime Parameters 

From the two sets of daily inflow data {i.e., historic and restored conditions), fovir separate flow regime 
characteristics were analyzed; including, event magnitude, duration, frequency and timing. First of all, event magnitude 
was used to indicate the amoimt of discharge from the Nueces River into the upper Nueces Delta during the 
period under consideration, and was determined by separating the estimated daily discharge values by period, 
and then averaging these by month. Next, event duration was used to express the cumulative length of flow 
events, and was determined by averaging the total number of days in which the stage of the Nueces River 
exceeded the flooding threshold (event days), regardless of discharge amounts. Also, e.v&at frequency was used to 
estimate the return period of peak daily flow events into the delta, and was determined by summing the number 
of events in each period that attained a given peak daily discharge amount, and then dividing this total by the 



Appendix C ♦ C-11 




2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 

Daily stage in the Nueces River at the Point of Diversion (ft msl) 



Figure 8: Stage-discharge rating curves for the Nueces River into the upper 
Nueces Delta, including both historic and restored conditions. Full range of 
stage values shown. 

Note: 1 acre-ft = 1.2335 10=' mM ft = 0.3046 m. 



(0 (B 

-a 3 
-oZ 
0) •- 

■S <i> 
m Q. 

E Q. 

S| 

o 



2000 

1800 

1600 

1400 

1200 

1000 

800 

600 

400 

200 





Historic conditions 

Restored conditions 








!5 30 35 40 45 50 55 60 65 70 75 80 8.! 

Daily stage in the Nueces River at the Point of Diversion (ft msl) 



Figure 9: Stage-discharge rating curves for the Nueces River into the upper 
Nueces Delta, including both historic and restored conditions. Only lower stage 
values shown to emphasize the rising and (average) falling limbs of each cun/e. 

Note: 1 acre-ft = 1.2335 10' m^ 1 ft = 0.3046 m. 



C-12 ^ Analysis of the Historic Flow Ke^me of the Nueces River into the Upper Nueces Delta 



Table 1: Comparison of estimated versus actual 
of Reclamation (2000), with modified falling limb 

during each event were used. 



discharge into the upper Nueces Delta using curves from Bureau 
curves. Only daily stage values greater than 0.76 m (2.5 ft) msl 



EVENT DATE 



Total 

Duration 

(days) 



Actual 

Discharge 

(acre-ft) 



Estimated 

Discharge 

(acre-ft) 



Deviation 
from Actual 



Maximum Stage 
Attained 
(ft msl) 



1997 



1998 



1999 



Jun 23 -Julio 


27 


1,657 


1,559 


-5.9% 


5.72 


Oct 9-17 


9 


466 


1,232 


153.5% 


5.74 


Mar 30 


1 


24 


26 


8.3% 


2.6 


Sep 5-21 


17 


614 


547 


-10.9% 


4.43 


Sep 25 -Nov 11 


48 


3,609 


3,888 


7.7% 


7.31 


Mar 27 - Apr 7 


12 


191 


201 


5.2% 


3.36 


Jun 30 - Jul 7 


8 


171 


130 


-24.0% 


3.22 


Aug 23 -Sep 13 


22 


988 


1,254 


26.9% 


5.81 



TOTAL 



7,740 



8,837 



14.2% 



Note: 1 acre-fl = 1.2336 10' mM ft = 0.3046 m 



number of years in the period (annual frequency). Daily peak values greater than 123 10^ m^ (100 acre-ft) were 
rounded to the nearest hundred, and those less than this were rounded to the nearest ten. From annual 
frequency, cvimulative frequency was then determined by incremental summation. The return period of peak 
flow events for a given magnitude in each period was then calcvilated as the inverse of cumulative frequency. 
Finally, event timing was used to identify seasonal and annual patterns in flow events, and was determined by 
Slimming daily discharge values by week for the entire period vinder review. 

RESULTS 

For pvuposes of comparison, the 60-year record under investigation was divided into three separate periods, 
each corresponding to the construction of a major reservoir in the basin (Table 2). 

Period I extends from January 1, 1940 to April 9, 1958 (approximately 18.3 years). During this period the only 
major regulating structure in the basin was La Fruta Dam on the Nueces River. The dam's mfluence on larger 
flood events in the watershed was limited because the storage capacity of the reservoirs capacity was relatively 
small to begin with, and this decreased significandy overtime due to sedimentation (City of Corpus Christi 
1990). Given the absence of data prior to this structure. Period I therefore represents approximate "baseline" 
conditions in the watershed with, minimal influences on stream flow from reservoir construction. 

Period II extends from April 10, 1958, when Wesley E. Seale Dam was closed, to May 17, 1982 (approximately 
24.1 years). Wesley Seale Dam was also constructed on the Nueces River just downstream of the La Fruta dam 
site, submerging and replacing it as the City of Corpus Christi's primary water supply. Once completed, the 
larger size of Wesley Seale Dam enabled it to more significandy affect flood events. Period II therefore 
represents an intermediate period of reservoir development in the watershed. 

Period III extends from May 18, 1982, when Choke Canyon Dam on the Frio River was declared substantially 
complete, to December 31, 1999 (approximately 17.6 years). The addition of Choke Canyon Dam's storage 
capacity to that of Lake Corpus Christi increased the total storage capacity in the basin to over 1,221,165 10' m' 
(990,000 acre-ft). For die latter part of this period (since June 24, 1997), Lake Corpus Christi was operated at 
an elevation of only 27.74 m (91.0 ft) msl (effective storage capacity of approximately 229,431 10' m' 
(186,000 acre-ft)) because of safety concerns. Period III therefore represents the climax period (or present 
conditions) of reservoir development and operation in the watershed. 

For each of these three periods, data from the largest flood event in that period was not considered in the 
analysis of flood event magnitude, frequency and duration for two primary reasons. First, these events were 
considered extra-ordinary, and therefore were not typical of the more frequent flow events of primary interest 



Appendix C ♦ C-13 



in this analysis. Second, the dutadon of 
each of the three periods was not deemed 
adequate to statistically appreciate the full 
dimensions of these events. For example, 
data for the flood event of 1967 (Period 
II), which is the largest on record in the 
past century, would have only been 
considered within a 24. 1 -year span, and 
would therefore have unacceptably skewed 
the results of that period, especially during 
the months in which the event occurred. 
Accordingly, the omitted flow events for 
Periods I, II and III were July 1942, 
September-October 1967, and June-July 
1987, respectively. ^\11 flow events, 
however, were included in the analysis of 
event timing, which is a more subjective 
measure where the fiill dimensions of each 
event are relevant for purposes of historical 
comparison. 



c 
o 



40 



30 - 



g. 
o 

0) 

i_ 
a. 

c 
c 
< 



n 20 



10 




1940-1957 1958-1981 1982-1999 



H Sabinal 

I I Corpus Christi 



19 Beeville 5 NE 
□ Cotulla 



Figure 10: Mean annual precipitation of available data at 
four gauges about the greater Nueces River watershed. 

Source: Medina 2000. Note: 1 inch = 2.54 cm 



It is interesting to note that Medina (2000) 

found the mean annual precipitation in the greater watershed during the first period (1940 through 1957) was 
consistendy the smallest when compared with the other two periods (1958 through 1981, and 1982 through 
1999) (Figure 10). The distribution of large precipitation events were also found to be less frequent during the 
drovight years of the late 1940's and early 1950's than in the latter two periods, which were not significandy 
different from each other (Medina 2000). This information is relevant because this first period, which was used 
as the baseline for calculating percent changes in delta inflow, likely under-represents, to some degree, the actual 
baseline conditions. 



Historical Freshwater Inflow into the Upper Nueces Delta 



Magnitude 

Historical event magnitude during Period I exhibited two primary peaks, one in the spring (May) and one in the 
fall (September), each at over 43,173 10^ m' (35,000 acre-fit) per month (Figure 11). Dtxring Period 11, event 
magtjitude also peaked twice during the year, but at lower discharge amounts. The spring peak Qune), which was 
less defined than in Period I, attained only about 8,635 10^ m^ (7,000 acre-ft), while the fall peak (October) 
attained almost 30,838 10^ m^ (25,000 acre-ft). During Period III, the trend of decreasing event magnitude was 
observed in extreme. No annual peaks were obvious, and the highest monthly average discharge amount was 
less than 370 10' m' (300 acre-ft) (May). 



Annual event magnitude represents the sum 
of all daily discharge values during the 
period divided by the period's duration in 
years. During Period II, or after the 
construction of Wesley Scale Dam, annual 
event magnitude decreased by 39'yo 
compared to Period I (Table 3). Since the 
construction of Choke Canyon Dam 
(Period III), the annual event magnitude of 
discharge events into the upper Nueces 
Delta decreased by over 99% from Period I. 



Table 3: Summary of historic annual event magnitude In the 
upper Nueces Delta. Mean discharge values rounded to the nearest 
10 acre-ft. 



Period 



1940-1958 
1958-1982 
1982-1999 



Historic 

Mean Total Discharge 

per Year 

(acre-ft) 



Percent Change 
from Period I 



128,000 

78,000 

540 



-39% 
-99.6% 



Note: 1 acre-ft = 1 2335 10^ m= 



C- 1 4 ^ Anafysis of the Historic Fhw Keffme of the Nueces River into the Upper Nueces Delta 



45000 

„ 40000 

it: 

b 35000 

O) 30000 
ra 

« 25000 



2 

(D 
> 
01 



20000 - 



15000 



10000 
5000 J 



1940-1958 
1958-1982 
1982-1999 



^V 



^' 



IV 



k 



K 



:^^ 



\.[ 



V 



V 



A^ 



Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 



Figure 1 1 : Historic magnitude of flow events into the upper Nueces Delta. Not 

included were data from the largest event in each time period. 

Note: 1 acre-ft = 1.2335 10' m' 




Jan 



Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 



Figure 12: Historic duration of flow events into the upper Nueces Delta. Not 

included were data from the largest event in each time period. 



Appendix C ♦ C-15 



100 



1940-1958 
1958-1982 
- 1982-1999 







^ 







.yy 


^"^ 

















10 100 1000 10000 100000 

Peak daily discharge into the upper Nueces Delta (acre-ft) 



Figure 13: Historic return frequency (ogive) of flow events into the upper 
Nueces Delta. Not included were data from the largest event in each time period. 

Note: 1 acre-ft = 1.2335 10^ ml 



300000 






o 
■o 

0) 

IS 

E 

LU 



100000 - 




Figure 14: Historic timing of 
flow events into the upper 
Nueces Delta. Data from all 
events in each time period 
included. Scale of the y-axis is 
curtailed from 700,000 acre-ft 
(September 1967) to improve 
resolution of lower values. 
Note; 1 acre-ft = 1.2335 10' ml 



C- 1 6 ♦ Anatfsis of the Historic Flow Regme of the Nueces River into the Upper Nueces Delta 



Table 4: Summary of historic annual event duration in the upper 
Nueces Delta. 



Diuation 

As with event magnitude, event duration under historical conditions also exhibited two seasonal peaks (Figure 
12). During Period I, the spring peak (June) and fall peak (September) attained about 5 average event days per 
month (5.0 and 4.8, respectively). During Period II, the spring peak (Jvme) was similar to that of Period I, but 
attained only 3.0 average event days per month, and the fall peak (October) about 4.2 average event days. In 
Period III, event duration also showed dramatic decreases, but unlike event magnitude, seasonal peaks were still 
discemable. The spring peak (May) peaked at about 1 . 1 average event days per month, and the fall peak 
(October) only about 0.5 average event days. During Period III, an anomalous third peak (Februar)') attained 
about 0.6 average event days, which reflects a disproportional influence of an (uncommon) winter event in 1992. 

Annual event duration represents the sum of 
all event days during the period divided by 
the period's duration in years. For Period I, 
event duration averaged about 21.1 average 
event days per year (Table 4). This mean fell 
to about 16.8 event days during the period 
after the closure of Wesley Seale Dam, and to 
about 4.5 event days during the period after 
the closure of Choke Canyon Dam. 

Frequency 

In general, the historical return period for event peak flows during Period I were slightly shorter than those in 
Period II (except for the largest values), but not appreciably (Figure 13). Period III, however, did exhibit a 
dramatic increase in the return period of event peak flows. 



Period 



1940-1958 
1958-1982 
1982-1999 



Historic 

Mean Number of Event 

Days per Year 



Percent Change 
from Period I 



21.1 
16.8 
4.5 



-20% 

-79% 



The difference in the return period of a flow 
event with a daily peak flow of 123.4 10^ m^ 
(100 acre-ft) between Periods I and II was 
an increase of less than a month (0.06 years) 
(Table 5). However, for the same event in 
Period III, the return period rose by over 
19 months (1.64 years). There was no 
return period for events with daily peak 
flows of greater than 617 lO'* m^ (500 acre- 
ft) during Period III. 

Timing 



Table 5: Summary of the historic return period for a flow event 
into the upper Nueces Delta with a daily peak flow of 100 acre-ft. 



Period 



I: 1940-1958 
II: 1958-1982 
III: 1982-1999 



Historic 

Return Period 

(years) 



Percent Change 
from Period I 



0.56 
0.62 
2.20 



11% 
293% 



Note: 1 acre-tt = 1.2335 10^ m^ 



In the three-dimensional presentation of event timing (Figure 14), the x-axis represents the calendar year in 
weeks, the y-axis represents the year of record and the z-axis represents total event discharge into the upper 
Nueces Delta. The seasonal peaks (spring and fall) observed in the magnitude and duration analyses were also 
manifest in event timing under historical conditions. As observed from the x-axis, spring flow events (May-June) 
were more frequent and smaller than fall flow events (September-October), which were more sporadic but 
generally larger during Periods I and II (Figure 13). This seasonal pattern, however, was noticeably absent during 
Period III. Winter (December-January) and svimmer (August) flow events for all periods were very rare. 

From the perspective of the y-axis, "wet" or "dry" periods in the delta were evident over the past 60 years. The 
more significant "wet" years for each period include 1941, 1942, 1946 and 1957, during Period I; 1958, 1967, 
1971, 1973 and 1981 during Period II; and 1987 and 1992 during Period III. Significant "dry" years in the delta 
include 1943, 1947, 1950, 1952, and 1955-56 in Period I; 1962-63, 1966, 1969, 1972 and 1978 in Period II; and 
all of Period III with the exception of 1987 and 1992. Except for an apparent decrease in the size of spring 
events during Period II, there was no obvious difference in flow event appearance between Periods I and II. 
There was, however, a marked difference in the comparison of flow events in Period III with either of the two 
preceding periods. The only discemable events in Period III were the summer event of 1987 (which would have 
been a much more significant event had not a large part of the flooding event served to fill and spill a nearly 
empty Choke Canyon Reservoir), and the late winter and spring events of 1992. Even so, these two events, 
while the largest in Period III, would be considered extremely small events in either of Periods I or II. 
Therefore, both the relative number and size of flow events in the delta contrasted starkly with the two 
preceding periods. 



Appendix C ♦ C-17 



Potential for Restored Freshwater Inflow into the Upper Nueces Delta 

One way of assessing the potential for how the Rincon Bayou Demonstration Project might restore freshwater 
flows to the upper Nueces Delta is to assiime that it had been in place in the past, and compare the results with 
what actually occurred. Event magmtude, duration, frequency' and timing were therefore analyzed assuming that 
the demonstration project features had been in place since the completion of Choke Canyon Reservoir 
(Period III), which was generally assumed to represent realistic future conditions. The results from this 
"restored" condition, when compared to those of the historical condition, would then give an indication of the 
restorative potential of the project. 

Magmtude 

The restored event magnitude for Period III, when compared with average monthly volumes that historically 
flowed into the upper Nueces Delta during Periods I and II, was almost imperceptible (Figure 15). The largest 
average monthly discharge amount (May) was only 1,208 10' m' (980 acre-ft), which represented only a fraction 
of the average for the same month during Periods I or II (2.5% and 16.8%, respectively). 



However, in the limited context of present 
conditions {i.e.. Period III only), the restored 
event magnitude compared much more 
favorably, increasing the estimated average 
monthly discharge by several rimes over 
what had occurred without the 
demonstration project. When analyzed 
annually, the restored event magnitude 
increased by over 633% from the historical 
Period III amount (Table 6). 

Duration 



Table 6: Summary of restored annual event magnitude In the 
upper Nueces Delta. Mean discharge value rounded to the nearest 
10 acre-ft. 



Period 



1982-1999 



Restored 

Mean Total Discharge 

per Year 

(acre-ft) 



Percent Change 

from Historic 

Period III 

Conditions 



3,420 



633% 



Note: 1 acre-ft = 1 2335 10^ m' 



Restored event duration, xmlike restored magnitude, compared very favorably with the historical duration of 
events flowing into the upper Nueces Delta (Figure 16). Both seasonal peaks were strongly evident, and the 
spring peak (May) was approximately the same duration as that of the Period I. The restored event duration 
even slightly exceeded the historical duration of both Periods I and II for seven (7) of the twelve (12) months of 
the average year, and in only two months was it lower than the historical Period II values. The only notable 
limitation of the restored event duration was that of the fall peak, which only attained an average of 2.5 days 
(October), compared to the historical fall peak of 4.8 days (September) in Period I and 4.2 days (October) in 
Period II. 



From the perspective of present conditions 
(Period III) only, the restored event 
duration also greatly surpassed historic 
levels (Table 7). The restored annual event 
duration increased by over 578% from that 
historical (without-project) level to 
26.0 days. 



Table 7: Summary of restored annual event duration in the upper 
Nueces Delta. 



Period 



1982-1999 



Restored 

Mean Number of Event 

Days per Year 



Percent Change 

from Historic 

Period III 

Conditions 



26.0 



578% 



Note: 1 acre-ft = 1 2335 10^ m^ 



C- 1 8 ^ Analysis of the Historic Flom Re^me of the Nueces River into the Upper Nueces Delta 






T3 
TO 
O 
<D 

> 

to 



45000 
40000 
35000 
30000 
25000 
20000 
15000 
10000 



■4= 5000 



1940-1958 
1958-1982 
1982-1999 
1982-1999* 




Jan Feb Mar Apr May Jun 



Aug Sep Oct Nov Dec 



Figure 15: Potential for restored flow event magnitude* into the upper Nueces 

Delta during Period III, which assumes the features of the Rincon Bayou 

Demonstration Project had been in place since 1982. Not Included were data 

from the largest event in each time period. 

Note: 1 acre-ft = 1.2335 10^ m^ 




Dec 



Figure 16: Potential for restored flow event duration* into the upper Nueces 
Delta during Period III, which assumes the features of the Rincon Bayou 
Demonstration Project had been in place since 1982. Not Included were data 
from the largest event in each time period. 



Appendix C ♦ C-19 



100 




100 1000 10000 100000 

Peak dally discharge Into the upper Nueces Delta (acre-ft) 



Figure 17: Potential for restored flow event return frequency (ogive)* into the 
upper Nueces Delta during Period III, which assumes the features of the Rincon 
Bayou Demonstration Project had been in place since 1982. Not included were 
data from the largest event in each time period. 

Note: 1 acre-ft = 1 2335 10^ m\ 




Figure 18: Potential for 
restored flow event timing* 
Into the upper Nueces Delta 
during Period III, which 
assumes the features of the 
Rincon Bayou Demonstra- 
tion Project had been In 
place since 1982. Data from 
all events in each time period 
included. Scale of the y-axis is 
curtailed from 700,000 acre-ft 
(September 1967) to improve 
resolution of lower values. 
Note: 1 acre-ft = 1.2335 10' m'. 



C-20 'J* Anafysis of the Historic Flow Re^me of the Nueces River into the Upper Nueces Delta 



Frequency 

The results of restored event return frequency Table 8: Summary of the restored return period for a flow event 
varied depending upon the peak flow into the upper Nueces Delta with a daily peak flow of 100 acre-ft. 

considered (Figure 17). For example, for Restored Percent Change from 

Period Return Period Historic Period III 



(years) Conditions 



events with peak daily flows below 49 10' m' 
(40 acre-ft), the estimated return period 

decreased to below historical levels of either III: 1982-1999 068 '324% 

Periodlorll. Above about 123 10' m' Note: 1 acre-ft = 12335 10' m' 

(100 acre-ft), the return period was greater than 

Periods I and II, but substantially less than that 

of Period III. If a peak flow event of 123 10' m' were used to illustrate the change in restored event return 

frequency from historical Period III conditions, the return period for this event would have decreased by about 

18 months, or by 324% (Table 8). 

Timing 

From inspection of Figure 18, and comparison with Figure 14, it is evident that the restored riming of freshwater 
flow events into the upper Nueces Delta, both in seasonal and annual distribution, would improve from Period 
III conditions. However, events xmder the restored condition would in no way attain the relative numbers or 
size of flow events that occurred during histoncal times. Each of the visible events imder restored conditions, 
although noticeably larger and longer, would still be considered only extremely small events in either historical 
Periods I or II. 

DISCUSSION 

Historical Freshwater Inflow 

Without exception, a decreasing trend over time was observed in each of the four historical flow regime 
characteristics analyzed. Event magnitude, duration, frequency and timing all declined with varying degrees from 
Period I through Period III. When compared to Period I, which represents a conservative reservoir scenario, 
flow regime characteristics during the period after the construction of Wesley Scale Dam (Period II) generally 
exhibited a lesser degree of change than did the period immediately after the completion of Choke Canyon Dam 
(Period III), which showed a more dramatic change. This would be expected, as Choke Canyon Dam provided 
over twice the amount of storage capacity of Lake Corpus Chtisti and was operated in conjunction with the 
latter, adding its effect upon the other. 

The magnitude of flow events during Period II decreased substantially from Period I levels, especially in regards 
to the spring event. Dxmng Period III, event magnitude was virtually eliminated when compared to previous 
periods. Changes in event duration closely reflected those in event magnitude, except that Period III levels did 
not decreased as significantly. The differences between the return period of flow events into the upper delta 
during Periods I and II were not as pronounced as with event magnitude and duration, but Period III again 
showed a significant departure from previous periods. Finally, event timing dramatically changed with the virtual 
elimination of meaningfiil seasonal flow events. This change was so distinct, that the entire post-Choke Canyon 
Reservoir period (with the exception of 1987) could be likened to a perpetual "dry" year, which occurred only 
infrequendy during the previous two periods. 

Potential for Restored Freshwater Inflow 

When compared with present conditions (represented by the historic Period III), each of the flow regime 
characteristics analyzed under the restored Period III conditions were substantially increased. j\nnual event 
magnitude was increased by over 633%, and annual event duration by over 578%. Event return frequency was 
also improved, and even exceeded that which occurred in Periods I and II for lower peak flow events. Finally, 
event timing also showed some improvement in the relative size and length of freshwater flow events. 

However, when compared with the historical flow regime of Periods I and II, the restored regime characteristics, 
with the exception of event duration, compared less favorably. The restored event magnitude was still only a 
fraction of even Period II levels, and event frequency was not affected for peak flow events greater than about 

Appendix C ♦ C-21 



1^34 10' m' (1,000 acre-ft). Also, improvements in event timing were not sufficient to eliminate the significant 
number of seasonal and annual "dry" periods in the Period III historical record. 

Comparison of Freshwater Flow into the Upper Nueces Delta Versus 
Inflow into Nueces Bay 

This analysis was focused exclusively on flow event characteristics into the upper (northern) Nueces Delta, and 
not on the total freshwater inflow into Nueces Bay. However, one minor digression worth consideration is the 
difference between changes in the historical freshwater inflow pattern of the upper deha compared with that of 
Nueces Bay. Clearly, flow into the upper Nueces Delta represents only a fraction of the river's total inflow to 
the receiving bays and estuary. Consequendy, the results of a historical analysis of total estuarine inflow, without 
consideration of the delta component, would be expected to differ considerably from the results of this 
investigation. 

And this is indeed the case. Asquith el al. (1997) evaluated the mean and median annual stream flow of the 
Nueces River near Mathis for the period 1940-1996, and performed statistical tests for historical trends with 
time. Their results indicated that the change in stream flow from 1958 through 1982 (or Period II) was 
negligible (an 0.8% decrease in mean annual flow, and an 18.5% decrease in median annual flow), while the 
change in stream flow after impoundment of Choke Canyon Reservoir was large (a 55.0% decrease in mean 
annual flow, and a 63.4% decrease in median annual flow). This conclusion is in contrast to the results of the 
present analysis, which indicate the decrease in annual mean volume of water entering the upper Nueces Delta 
during Period II was considerable (about a 39% reduction), and during Period III was extreme (over a 99% 
reduction). 

This pragmatic explanation of this difference between the historic flow characteristics of the upper Nueces Delta 
and those of the greater Nueces Bay Hes in the concept of "flooding threshold" at the point of natural diversion. 
Because of the higher threshold imposed by the elevated river bank, freshwater diversions into the delta system 
are extremely sensitive to the peak segments of flood events in the Nueces River. Therefore, if the flooding 
threshold is not met, the flow event in the river will bypass the delta, providing Nueces Bay with an freshwater 
inflow event without the same courtesy for the delta. Hence, changes in river flow patterns which lower the 
peak flows of flood events disproportionately affect the upper delta as compared to the bay. 

The value of the distinction between delta inflow and total estuarine inflow is fully appreciated when recognizing 
the fact that the Nueces Delta is a distinct and critical component of the greater estuary system. Without 
consideration of this point, one may erroneously conclude that, for example, the reductions in freshwater inflow 
during Period II did not meaningfully alter the freshwater flow regime of the Nueces Estuary ecosystem because 
total mean inflow into Nueces Bay was reduced by only one percent. 

CONCLUSIONS 

The historic flow regime of the Nueces River into the upper Nueces Delta has changed dramatically over the 
past 60 years. In each of the flow event characteristics analyzed, a strong declining trend was observed from 
Period I through Period III. The Rincon Bayou Demonstration Project would restore a considerable amount of 
freshwater to the upper Nueces Delta and significandy improve the flow regime characteristics compared to the 
present (historical Period III) conditions. However, the demonstration project would not restore the delta's 
inflow patterns to historic (Periods I and II) levels. 

Reservoirs in the basin and deltaic inundation events have a imique relation to peak flow events in the lower 
Nueces River. On one hand, because of the high flooding threshold of the north bank of the river, the upper 
Nueces Delta relies almost exclusively on the peaks of flow events for freshwater inflow. On the other hand, 
main-stem reservoirs, by design, significandy attenuate the peaks of flow events in the watershed for purposes of 
water storage and flood control. This relation is exemplified by the fact that, during the period after the 
construction of Lake Corpus Christi (Period II), although annual mean flow of the river into Nueces Bay was 
reduced by about 1% (Asquith et al. 1997), the annual mean flow into the upper Nueces Delta was reduced by 
about 39%. Similarly, during the period after the construction of Choke Canyon Dam, while the annual mean 
flow of the Nueces River into the bay was reduced by about 55% (Asquith et al. 1997), the annual mean flow into 
the upper delta was all but eliminated (reduced by 99%). 



C-22 ^* Anatjists of the Historic Flow Eiffme of the Nueces River into the Upper Nueces Delta 



Therefore, if the upper deha's historical contribution to the estuary ecosystem is to meaningfully exist in the 
future, the multi-component flow regime of the Nueces River into the upper Nueces Delta must be considered 
in the context of reservoir operation and management. 

LITERATURE CITED 

Asquith, W.H., J.G. Mosier, and P.W. Bush. 1997. Status, Trends and Changes in Freshwater Inflows to Bay 

Systems in the Corpus Christi Bay National Estuary Program Study Area. CCBNEP-17. Coastal Bend Bays 

and Estuaries Program, Corpus Christi, Texas. 
Bureau of Reclamation. 1971. Nueces River Project, Texas: Appendixes to Feasibility Report, Volume I- 

Hydrology. United States Department of the Interior, Bureau of Reclamation. 
Bureau of Reclamation. 1996. Hydrauhc analysis of Rincon Bayou using HEC-2. United States Department of 

the Interior, Bureau of Reclamation, Billings, Montana. 
Bureau of Reclamation. 2000. Hydrauhc Analysis of Rincon Bayou Using HEC-2, Rincon Bayou-Nueces Marsh 

Wetlands Restoration and Enhancement Project. United States Department of the Interior, Bureau of 

Reclamation, Great Plains Regional Office, Billings, Montana. 
Corpus Christi. 1990. Annual report for fiscal year 1989-1990, City of Corpus Christi Water Division. City of 

Corpus Christi, Texas. 
Corpus Christi Times. 1967. Article from September 26 edition entided 'TSIueces River Crest Past; Beach is 

Safe". Corpus Christi, Texas. 
Gandara, S.C., W.J. Gibbons, F.L. Andrews, R.E. Jones and D.L. Barbie. 1997. Water resources data for Texas, 

water year 1996. United States Geological Survey, Austin, Texas. 
HDR Engineering, Inc., Shiner, Moseley & Associates, Inc. and Naismith Engineering, Inc., in association with 

University of Texas, Marine Science Institute. 1991. Regional water supply planning study: Phase I, Nueces 

River Basin, Volume II. HDR Engineering, Inc., Austin, Texas. 
Hilzioger, L. 2000. Personal communications. City of Corpus Christi, Water Supply Department, Wesley Scale 

Dam. 
Homer & Shifiin. 1951. Review of the Corpus Christi water supply problem. Homer & Shifrin Consulting 

Engineers, under an association with Harland Bartholomew & Associates, St. Louis, Missouri. 
Karr,J.R. 1991. Biological integrity: A Long-neglected aspect of water resource management. Ecologcal 

Applications 1 : 66-84. 
Longley, W. L., ed. 1994. Freshwater inflows to Texas bays and estuaries: ecological relationships and methods 

for determination of needs. Texas Water Development Board and Texas Parks and Wildlife Department, 

Austin, Texas. Page 386. 
Medina, J.G. 2000. Recent trends in precipitation occurring on the Nueces River watershed of south Texas. 

Unpubhshed. In Concluding Report: Rincon Bayou Demonstration Project, Appendix D. United States 

Department of the Interior, Bureau of Reclamation, Austin, Texas. 
Poff, N. LeRoy, J. and J.V. Ward. 1989. ImpUcations of streamflow variabUIity and predictabiUty for lotic 

community structure: a regional analysis of streamflow patterns. Canadian Journal of Fisheries and Aquatic 

Sciences \(i: 1805-1818. 
Poff, N. LeRoy, J. D. Allen, M.B. Bain, J.R. Karr, K.L. Prestegaard, B.D. Richter, R.E. Sparks and J.C. 

Stromberg. 1997. The natural flow regime: A paradigm for river conservation and restoration. BioScience 

Vo. 47, No. 11, pp. 769-784. 
Richter, B.D., J. V. Baumgartner, J. Powell, and D.P. Braun. 1996. A method for assessing hydrologic alteration 

within ecosystems. Conservation Bioloff/ 10: 1163-1174. 
Salas, D.E. 1993. Vegetation assemblage mapping of the Nueces River delta, Texas. Unpubhshed. United 

States Department of the Interior, Bureau of Reclamation, Denver, Colorado. 
Texas Department of Water Resources. 1981. Nueces and Mission-Aransas Estuaries: A study of the influence 

of freshwater inflows. Texas Department of Water Resources, LP-108, Austin, Texas. 
Texas State Highway Department. 1931. Plans for proposed Nueces River and rehef bridges. Highway No. 9, 

Nueces and San Patricio Counties. Texas State Highway Department, Corpus Christi, Texas. 
Texas State Highway Department. 1956. Plans of proposed State highway improvement. State highways 9 and 

77, Nueces and San Patricio Counties. Texas State Highway Department, Corpus Christi, Texas. 
Texas State Highway Department. 1959. Plans of proposed State highway improvement. Interstate Highway 37, 

San Patricio County. Texas State Highway Department, Corpus Christi, Texas. 
Texas State Department of Highways and Public Transportation. 1983. Plans of proposed State highway 

improvement. Interstate Highway 37, San Patricio County. 
United States Engineer Office. 1939. Report on survey of Nueces River and tributaries, Texas, for flood control 

and allied purposes. United States Engineer Office, Galveston, Texas. 



Appendix C ♦ C-23 



United States Geological Survey. 1968. Water delivery study, lower Nueces River valley, Texas. Report 75. 

Texas Water Development Board, Austin, Texas. 
Walker, K.F., F. Sheldon and J.T. Puckridge. 1995. A perspective on dryland river ecosystems. Regtlated Rivers: 

Research i& Management \\: 85-104. 
Ward, G.H. 1985. Marsh enhancement by freshwater diversion, journal of Water Resources Planning and 

Management, ASCE, 111 (1), pages 1-23. 



C-24 ^ Analysis of the Historic Flow Regme of the Nueces River into the Upper Nueces Delta 



APPENDIX D 



Recent Trends in Precipitation 
Occurring on the Nueces River 
Watershed of South Texas 



Jonnie G. Medina Meteorologist, U.S. Department of the Interior, Bureau of Reclamation, P. O. Box 
25007, Code D-3720, Denver, CO 80225. 

ABSTRACT: Precipitation from the Nueces River watershed was analyzed for recent trends as part of the 
Rincon Bayou Demonstration Project (Rincon Project) conducted by the United States Bureau of Reclamation. 
Of interest were comparisons of precipitation across the three periods, 1940-57, 1958-81, and 1982-99, of 
interest to the Rincon Project, a demonstration and study of impacts on the ecology of the upper Nueces delta 
Bay from enhanced inundations of the marsh. The role is discussed of the EI Nino-Southem Oscillation 
(ENSO) and other climate variables to tropical cyclone occurrence in the Atlantic basin and in particular, the 
Gulf of Mexico. The potential modulation by ENSO of watershed precipitation was investigated. A 
multivariate comparison was conducted of daily precipitation sampling distributions across project study 
periods. The occurrence of large precipitation events was of particular interest because they could lead to 
natural inundations of the marsh. Analyses indicated no particular watershed precipitation trend unless the 
basis of comparison was the extreme drought period of the 1950s. The La Nina (El Nino) phase does appear 
to lead to more (less) tropical cyclone impacts in the western Gulf of Mexico than baseline years. Of 21 years 
(since 1948) with large precipitation events (=> 4 inches per day) impacting Corpus Christi, 6 were El Nino 
years, 8 were La Nina years and 7 were baseline years. Tropical cyclones occurred in 7 of the 21 large-event 
years; 5 were La Nina years and 2 were baseline years. Of 7 years with annual precipitation exceeding 
40 inches, 5 years were baseline years, 1 year was La Niiia, and the largest-amount year was El Nino. Two of 
the seven highest years did not have a high daily event. The multivariate comparison revealed the sampling 
daily precipitation of the drought years differed from the other two study periods, otherwise no differences. 

INTRODUCTION 

The United States Bureau of Reclamation (Reclamation) is sponsoring and partnering in the Rincon Bayou 
Demonstration Project (Rincon Project) aimed at determining marsh response to increased freshwater 
inundation caused by construction of an overflow channel on the Nueces that flows into Nueces Bay just 
north of Corpus Christi, Texas (see Figure 1). The occurrence of natural over-banking events from the Nueces 
River into the Nueces Delta was reduced by the construction in 1958 of Lake Corpus Christi on the Nueces 
River and in 1982 by Choke Canyon Reservoir on the Frio River (Irlbeck and Ward 2000). In addition to 
regulated flows into the Nueces River, precipitation in the Frio and Nueces watersheds, and locally in the delta, 
also impact the delta ecosystem. The question arises as to how much, if any, of the observed decreases in the 
magnitude, frequency, duration and timing of delta inundation events since 1958 (Idbeck and Ward 2000) has 
been caused by variations in climate. 

The weather systems impacting south Texas migrate from the temperate zone to the north or the tropics. Some 
disturbances originate in southwestedy or westerly flow over Mexico. Tropical disturbances approach the 
Nueces watershed generally from the southeast or east. Except in circumstances of deep continental flow, the 
troposphere over south Texas generally contains ample moisture for precipitation, but often lacks a trigger 
mechanism to build storm clouds. Perturbations moving toward the Rincon area generally provide the triggers. 
For example, a slow-moving front or tropical wave can trigger the development of clouds that can produce 
small amounts to several inches of precipitation, often in isolated patterns rather than general coverage. So, a 
weather system may produce littie precipitation at a location, but a short distance away can produce three inches 



Appendix D ♦ D-1 




Figure 1: Nueces River Basin and precipitation gauges. 



Trends in precipitation should be described within an a priori selected time period. For example, a short-term 
trend following a drought would be increasing precipitation. The duration of such trends is of great interest 
and the ability to forecast them is constandy sought. For purposes of the Rincon project, precipitation trends 
over 1940-99, a 60-year period, are the main concerns of study. Additionally, Rincon area precipitation is 
compared among the periods 1940-57, 1958-81, and 1982-99, that correspond to different construction 
influences on Nueces River flows. 

On the National basis, the Climate Prediction Center (CPC) of the National Oceanic and Atmospheric 
Administration routinely develops temperature and precipitation trend estimates (available on the Internet, 
address ftp://ftp.ncep.noaa.gov/charts.htinl/') . CPC uses 102 climate regions of near equal area for the lower 
48 states. Each climate region is composed of one or more climate divisions. Monthly data are assembled into 
time series for three-month periods and an annual average. Baseline values were developed from data to 1966. 
The linear change per decade is computed from the base line value through the current year. Current trends are 
based on 1941-1998 data for temperatures and 1931-1998 data for precipitation. The CPC trends indicate 
National increases of 0.15 °F and 0.9 inches of precipitation per decade. For south Texas, no trend in mean 
annual temperatures is indicated, but a 0.2 to 0.6 inch increase in precipitation per decade is revealed. 

The following covers a discussion of precipitation in the Nueces River watershed based on data from several 
gauges located there. A summary is given of some study results on the El Nino-Southem Oscillation (ENSO) 
and other climate variables, and their relationship to tropical cyclone occurrence in the Atlantic basin and in 
particular, the Gulf of Mexico. Nueces River watershed precipitation anomalies stratified by ENSO type are 
presented. Finally, daily precipitation distributions for one watershed gauge are developed and discussed in the 
context that for different periods there can occur different precipitation distributions causing differing runoff 
while maintaining s imilar annual precipitation. 

NUECES RIVER WATERSHED PRECIPITATION SURVEY 

The 30-year mean precipitation for gaviges shown in Figure 1 portray differences across the Rincon area. Mean 
annual preapitation (1961-1990 record) for Corpus Christi WSO AP, CotuUa, Carrizo Springs, BeeviUe 5 NE, 
and Sabinal are the respective 30.14, 22.95, 21.22, 31.97, 25.45 inches. These values support lesser 
precipitation occurs in the southwestern Rincon area and higher amounts along the coastline. Figures 2 
through 5 present annual precipitation totals for Cotulla, Corpus ChristL, Beeville 5 NE, and Sabmal through 
1999, with a Lowess (Cleveland, 1979; 1981) smoothing line (average of weighted values in a moving window 



D-2 *♦♦ Recent Trends in Precipitation Occurring on the Nueces River Watershed of South Texas 



that allows adjustable flex by changing the window size) that suggests trends. These precipitation gauges are 
distributed across the Rincon Project drainage suppljnng the delta and possess the better quahty data for that 
area. Annual precipitation recording began shordy after 1900, except for the Corpus Christi record that began 
in 1948. The figures show the prominent drought of the 1950s (and several years of the late 1940s). The 
Lowess cixrves show short-term recent rising precipitation at three of the four gauges. The Corpus Christi 
Lowess curve portrays an unrealistic initial pattern because of analysis effects from initial data from the 
drought. The inclusion of the drought years to the CPC baseline largely caused the positive trend in 
precipitation. 

Line plots (not given) of the Corpus Christi WSO AP, Sabinal, and Beeville 5 NE gauges produced respective 
positive, negative, and positive slopes using the available annual precipitation for each gauge. The Corpus 
Christi WSO AP data yields a negative slope if precipitation is restricted to 1965 or more recent toformation. 
The Beeville 5 NE precipitation produces a negative slope if restricted to 1960 or more recent. 

Asquith et al (1997) conducted a study to determine the ctirrent mean freshwater inflows to bay systems that 
included the Corpus Christi Bay. As part of their study, they examined temporal trends in inflows. They 
studied trends in precipitation of several precipitation gauges. Asquith et al (1997) applied the Mann-Kendall 
test (Helsel and Piirsch, 1992; Hollander and WoLfe, 1973) to time series of seasonal and annual rainfall from 
the Corpus Christi WSO AP and Refugio 7 North (located approximately 30 miles east of the Beeville 5 NE 
gauge) gauges. The Mann-Kendall test is a rank-based nonparametric test similar to the familiar Wilcoxon- 
Mann-Whitoey procedures to test for monotone increasing character in data. Results of the statistical analysis 
indicated P- values not significant at the 0.05 level in all seasonal and annual time series of 1968-93 data for 
Refugio 7 North, and similarly for Corpus Christi WSO AP data excepting winter (January, February, March) 
that produced an upward trend with a P-value of 0.05. The annual Corpus Christi data produced a 0.25 
P-value. 

Figure 6 presents the mean annual precipitation computed separately for the four gauges, Sabinal, Corpus 
Christi, Beeville 5 NE, and Cotulla for each time period of interest to the Rincon Project. This portrayal of 
gauge mean precipitation shows the change across time periods and gauges. Because the Corpus Christi record 
began in 1948, the first period duration was restricted to 10 years; other period lengths were 24 and 18 years. 
The figure shows all gauges displaying the same general pattern over the three periods, least precipitation during 
the drought period, greatest amount during the second period, and intermediate amounts during the third and 
most recent period. 

Precipitation in the Nueces watershed is not monotonic increasing over the three study periods. However, 
there could be patterns of rainfall amounts that could cause more inundations of the Nueces Delta than other 
patterns with similar annual precipitation. Namely, the annual precipitation could occur in relatively few large 
amoimt events. This could occur from increased tropical disturbances numbers, more persistent frontal 
patterns that favor precipitation in the watershed, or perturbations in a persistent moist Pacific flow across 
northern Mexico and the southern United States (shown in Figure 9). 

Figures 7 and 8 present cumulative adjusted annual precipitations over the gauge record periods for the Sabinal 
and Corpus Christi WSO AP gauges (through 1998), respectively. Annual precipitation values were adjusted by 
removing the long-term annual mean. Difference values were then summed in time and plotted. The 
cumulative precipitation differences are related to soil moisture. Though such cumulative difference figures are 
impacted by the choice of data starting point, the figures for the Rincon Project study area portray the 
substantial annual precipitation deficit created by the severe drought of the late 1940s and 1950s. The longer- 
term Sabinal record shows the contrasting conditions prior to and after the 1950s. The plot for Sabinal 
suggests that the upper Nueces River basin may have frequendy been in below-normal soil moisture conditions. 

ENSO AND TROPICAL CYCLONES IN THE ATLANTIC BASIN 

The relative maximum in precipitation that occurs in the fall season in the Nueces River watershed is largely the 
result of, (1) mid-latitude perturbations that in fall strengthen, adequate to reach south Texas as the subtropical 
and polar jets reestablish in the stronger north/south temperature gradient, and (2) tropical perturbations that 
impact the western Gulf of Mexico. Tropical cyclones can substantially impact western Gulf precipitation. 
Landsea et al. (1999) studied Atlantic hurricanes. United States (lower 48 States) land-falling hurricanes and U.S. 
normalized damage (adjusted for changes in inflation, coastal county population, and wealth) time series for 

Appendix D ♦ D-3 



50 



40 



c 
o 



c5 30 



a. 
o 

0) 



20- 



"1 — 1 — I — 1 — I — 1 — I — I — r 



»• •* • . . 



J I I I ■ I L 



\ • 



10 

s<i^^ ^^^° ^<i^° ^.i^ ,9^° ^^^ ^<,^ ^<»A° ,# ^^ ^OCP 

Year 

Figure 2: Cotulla annual precipitation. 



50 



40 



c 
o 

ffl 30 






20 



10 



1 I I I I 1 r 




J I I I L 



J L 



^<i^ ^9^° ,<»T-^ ,<i^° ,.»^° ,<i^^ ^<^ ,9^^ ,<i%^ ^^^ ^^ 

Year 

Figure 3: Corpus Christi WSO AP annual 
precipitation. 




50 



40 



c 
o 



« 30 



Q. 
O 



V V V V V V V V \^ V 
Year 
Figure 4: Beeville 5 NE annual precipitation. 



20 



10 



"I I — I — I — I — I — r 



-• ^ 



J L 



J L 



^<i^ ^<J^^ ^<iT? ^ci^"" ,<.^° <^^ ^c,^ ^<i^^ ^^«P ^q<^° TpO° 

Year 
Figure 5: Sabinal annual precipitation. 



35 



c 30 
o 



a 25 



o. 
^ 20 



15 






M Sabinal 

Bl Corpus Christi WSO AP 

n Beeville 5 NE 

H Cotulla 



12 3 

PERIOD 



Figure 6: Mean annual precipitation per each time period of 1940-57, 
1958-81, and 1982-99. 



D-4 ♦♦♦ Recent Trends in Precipitation Occurring on the Nueces River Watershed of South Texas 



Cumulative Adjusted Precipitation 




Figure 7: Sabinal cumulative annual precipitation that was 
adjusted by subtracting the long-term mean. A dotted vertical line 
is placed at 1982 indicating the beginning of the Rincon third study 
period. 



Cumulative Aa^usted Frecipitation 
lOi 




-90 



1940 1950 1960 1970 1980 1990 2000 
Year 



Figure 8: Same as figure 7 except for Corpus Christi Gauge 
information. 



Appendix D ♦ D-5 



inter- annual trends and multi-decadal variability. They used records on tropical disturbances back to the 1 940s 
for basin-wide Atlantic cases, and the turn of the 20* cenfur)' for U.S. land-falling systems. \'arious 
environmental factors were considered including Caribbean sea level pressures, 200-niillibar zonal winds, 
stratospheric Quasi-Biennial Oscillation (described below). El Nino-Southem Oscillation (ENSO, described 
below), African West Sahel rainfall and Atlantic sea surface temperatures. AH variables indicated significant, 
concurrent relationships to the frequenc}', intensity and duration of Adantic hurricanes. The ENSO was found 
to be significandy linked to changes in damages by tropical cyclones that impact the United States. 

Gray et al (1993) found that various relationships can be determined if data are stratified into spatial location 
and by disturbance intensity. The linear trend in hurricane activity of the Atlantic basin is vety weak (Landsea 
et al., 1999). Multi-decadal variabilitj' is more characteristic of the region. This is suggested in that intense 
Atlantic hurricanes (50 m/s or more) were common in the 1940s through the 1960s, but much reduced from 
the 1970s through the early 1990s (Landsea, 1993). In 1995 and 1996, there occurred a return to high levels of 
tropical cyclone activity s imil ar to earUer active decades. There were similarities in hurricane dilation variations 
with longer-lived systems (about 25-40 tropical cyclone days per year) in the 1 940s through the 1 960s and fewer 
hurricane days (around 10-25 days per year) in the decades of the 1970s through eady 1990s. 

While the Atlantic basin exhibits multi-decadal variation, the United States Gulf Coast from Texas to the 
Florida panhandle expresses much weaker intense hurricane strike variability (Landsea et al, 1999). The 
variation pattern is quite different as above average acti\'ity occurred only in the 1910s and reduced activity only 
in the late 1940s and early 1950s. Thus, some components of the Adantic basin may exhibit markedly different 
intense hurricane variation. 

ENSO events induce moderate-sized changes in the frequency and intensity of Adantic basin tropical cyclones 

(Landsea et al, 1999). The ENSO state can be characterized by the sea surface temperature (SST) anomalies in 
the eastern and central equatorial Pacific (Philander, 1989). Warm SSTs in this region are referred to as El Nino 
events, and cool SSTs are known as La Niria events. The state of ENSO can also be characterized by the 
Southern Oscillation Index (SOI), the standardized difference in sea level pressiure between Tahiti and Darwin, 
Australia. High (low) pressure at Darwin and low (high) pressure at Tahiti corresponds to El Nino (La Nina) 
events. 

Figures 9 and 10 present schematics 

of the winter tropospheric jet 

streams and major 

cyclone/anticyclone features of the 

El Niiio and La Nina events, 

respectively. The schematics show 

the El Nino Pacific jet stream 

transporting moisture into the 

southern States thus favoring wetter 

conditions there, while under the La 

Nina structure the storm track is 

fiirther north. ENSO events alter 

the global atmospheric circulation 

patterns and are able to affect 

tropical cyclone frequencies. The 

mechanisms for the latter are the 

alteration of the lower tropospheric 

source of vorticity (measure of 

rotation in a fluid) and the vertical 

wtad shear profiles (Gray, 1968, 

1979). ENSO fluctuates on the scale of a few years (Philander 1989) 




Figure 9: El Nifio winter atmospheric features. 



El Niiio events are associated with fewer numbers of Adantic basin tropical cyclones (recall that El Nino leads 
to more nontropical storms particularly during the cool season). La Nina events cause 36 percent more 
named/subtropical storms than El Niiio events. Their mean intensities are also 6 percent stronger (Landsea et 
al, 1999). Though ENSO events modulate hurricane landings in the Caribbean region and the United States, 
the impacts are only weakly significant for the Gulf Coast intense hurricanes (and not significant for weaker 
storms). 



D-6 V Recent Trends In Precipitation Occurring on the Nueces River Watershed of South Texas 



For intense humcane strikes along the Gulf 
Coast, the single largest factor of influence 
is the phase of the Quasi-Bieimial 
Oscillation (QBO; Gray 1984, Shapiro 
1989). The QBO is an east-west oscillation 
of stratospheric-level winds that encircle 
the globe near the equator. In the west 
phase, Atlantic hurricane activity is 
enhanced, but is diminished in the east- 
phase QBO years. The QBO is followed in 
importance by the 200-millibar zonal winds 
and sea-level pressure anomalies, such that 
westedy winds and high pressures in the 
Caribbean favor more tropical cyclone 
landings (Knaff, 1997). With regard to 
interannual variation, normalized United 
States' damages are well related to ENSO, 
such that significantly less damage occurs 
during El Nino events and than dvuing 
La Nina events. 



JET "^ . 




Figure 10: Winter features for La Nina. 



Lack of hurricane variation consistency in the Gulf of Mexico compared to East Coast occurrences is likely 
caused by the dominance of local conditions over basin-wide factors that can lead to intense hurricane 
development (Landsea et al, 1999). An example is tropical storm development along a stationary frontal 
boundary of strongly contrasting conditions. This occurred in the case of hurricane Alicia in 1983. The 
prominent role of local conditions is likely why there is no distinct multi-decadal variation of intense hurricanes 
in the Gulf of Mexico (Landsea et al, 1992). 

Tropical cyclone activity in the western Gulf of Mexico during the three periods of interest to the Rincon 
Project somewhat followed the causal/correlated patterns discussed previously. Table 1 gives tropical cyclone 
occurrences by ENSO type and baseline (non-ENSO) years. Classification of years was according to PieLke and 
Landsea (1999). More tropical cyclones impacted the western Gulf of Mexico in La Nina years than El Nifio 
years, generally following the script. However, table 1 shows that six tropical cyclones impacting the western 
Gulf occurred during the relatively low Adantic basin activity period of the 1970-80s. In the first Rincon study 
period (1940-57), only hurricanes' Alice and Audrey affected the western Gulf of Mexico. During the second 
study period (1958-81), 11 tropical cyclones impacted the Gulf coast. In the third study period (1982-99), four 
hurricanes (Arlene, Charley, Frances and Bret) occurred, three during El Niiio years and the fourth (Bret) 
during the La Nina year of 1999. 

WATERSHED PRECIPITATION ANOMALIES AND ENSO PHASE 

Table 1 shows twice as many tropical cyclones occurring in the western Gulf of Mexico during La Nina years as 
El Nino years. The question arises as to whether Nueces River watershed precipitation is sensitive to ENSO 
phases. For example, do La Nina months record more precipitation baseline-year months. To investigate this 
issue, monthly precipitation for 1950-99 for Corpus Christi WSO AP, BeeviUe 5 NE, and Sabinal were stratified 
into baseline (16 years and 4 tropical cyclones). El Nino (18 years and 4 tropical cyclones), and La Niiia years 
(16 years and 8 tropical cyclones). All months for a particular category (ENSO, baseline) were used in analyses 
to capture the effects on precipitation of nontropical storms as well as tropical cyclones. For each precipitation 
gauge, the 75 percentile precipitation was determined in the base years to serve as the benchmark of 
comparison. The analysis technique applied, similar to Gershunov and Bamett (1998), determined the 
proportion of months of record with a precipitation amount that equaled or exceeded the benchmark. The 
proportion determined is compared to 0.25 and the quotient is reduced by 1.0 and converted to percent 
anomaly. 



Appendix D ♦ D-7 



Table 1: Tropical cyclones during ENSO and baseline years. 



El Nifio Years 


La Nina Years 


Baseline Years 


1940 


1942 


1943 


1941 


1944 


1946 


1951 


1945 


1947 


1953 


1948 


1952 


1957 (Audrey) 


1949 


1958 


1963 


1950 


1959 


1965 


1954 (Alice) 


1960 (TS1) 


1969 


1955 


1962 


1972 


1956 


1966 


1976 


1961 (Caria) 


1968 (Candy) 


1977 


1964 


1979 


1982 


1967 (Beulah) 


1980 (Allen, Danielle) 


1986 


1970(Celia) 


1981 


1987 


1971 (Fern) 


1983 


1990 


1973 (Delia) 


1984 


1991 


1974 


1985 


1993(Arlene) 


1975 


1989 


1994 


1978 (Amelia) 


1992 


1997 


1988 


1996 


1998 (Charley, Frances) 


1995 
1999 (Bret) 





Table 2 lists the anomalies calculated. It is noted that the baseline months included the effects on precipitation 
from four tropical cyclones. Additionally, because proportions developed use counts of months rather than 
precipitation amounts, extreme events cannot excessively and unreahsticaUy skew results. The results reveal a 
mixture with El Nino monthly precipitation ranging from a (positive) 1.3 percent anomaly in the BeeviUe 5 NE 
data to a -30.7 percent anomaly in the Corpus Christi data. La Niiia anomalies ranged from -12.7 to 25.9 
percent, also a mixed result. Four of six ENSO anomahes were negative. Both ENSO phases led to negative 
anomalies at the Corpus Christi gauge, but a smaller anomaly (-12.7 percent) for La Nina months that included 
the effects from 8 tropical cyclones. The inland gauge, Sabinal, yielded the same pattern of a more positive 
anomaly from La Nina months, but this pattern was not maintained in the Beeville 5 NE data. NX'hile the 
Beeville 5 NE and Corpus Christi WSO AP gauges yielded the same anomaly for La Nifia months, there was a 
large difference in the El Nino related anomaly despite being separated by only about 40 miles (however, 
Beeville 5 NE is inland about 35 miles). There is a "weak" sviggestion that ENSO events may lead to a 
somewhat higher proportion of negative anomalies than expected by randomness (discounting the zero 
anomaly, there is a 23 percent probability of obtaining 4 negative anomahes in 6 possibihties). More data from 
additional gauges would be needed to further explore this possibility. Generally, these analyses in search of a 
precipitation dependence on ENSO do not reveal any well-defined pattern, but rather expressed the high 
variability in the precipitation data. 

Table 2: Percentage anomaly of monthly ENSO rainfall when compared to base year 75 percentile 
benchmark precipitation. 



Beeville 5 NE 
La Nina: -12.7% 
El Nino: +1.3% 



Sabinal 
La Nina: 
El Nino: 



+25.9% 
-6.8% 



Corpus Christi 
La Nina: -12.7% 
El Nino: -30.7% 



DAILY PRECIPITATION DISTRIBUTION COMPARISONS 

While there appears litde indication of a trend in the annual and monthly precipitation of the Nueces River 
watershed during the 1940 to 1999 period, there remained the possibility that there could have been a 
difference in the distribution of daily precipitation across the three periods of interest in the Rincon Project 
study. For example, the mean monthly precipitation during the second period could have been similar to that 



D-8 ^* Recent Trends in Precipitation Occurring on the Nueces River Watershed of South Texas 



during the third period but a differing daily amount distribution could cause highly differing delta inundations 
in one period versus the other period. One approach to study this possibility is the cross-tabulation of daily 
precipitation. The Corpus Christi WSO AP daily precipitation was selected for analysis because of better 
quality, despite the shorter record period that initiated in 1948. 

Given in table 3 are the frequencies from cross-tabulation of the Corpus Christi daily precipitation in 
approximately one-inch size categories. Inspection of the frequency patterns over time su^ests some 
differences in shorter time periods of about 10 years (particularly the first 10 years). The number of years per 
each of the 3 Rincon Project periods in succession was 18, 24 and 18 years. Available Corpus Christi WSO AP 
data limited the initial study period to 10 years (1948-99). Cross-tabulations present outcomes such that pattern 
comparisons for selected periods can often be made using muUtvariate analysis. A distribution-free 
permutation test was selected and applied in exploratory analysis to explore whether the three distribution 
patterns differed. The statistical test applied is one of a family of tests known as multi-response permutation 
procedures (MRPP) (Mielke et al., 1976, 1982). These tests avoid making the unjustified assumption that the 
joint distribution of the response measurements is multivariate normal or some other specific distribution. 

Application of the MRPP yielded a P-value of 0.08 in comparing frequency patterns of all three periods. In the 
exploratory sense, the first 10 years of frequencies were separately compared with those of each of the 24 and 
18 year periods using MRPP, and ia both applications found to differ at better than the 5 percent level. 
Application of MRPP in a comparison of frequencies of the 24 versus 1 8 year periods was not significant at the 
5 percent level. In this exploratory analysis, no attempt was made to correct for experimentwise error rate 
under a partial null hypothesis in which some comparisons are equal but others differ (Miller, 1981). The 
results of these comparisons are not surprising given the extreme level of drought in the 1950s. Additionally, 
shorter comparison periods, such as the 10-year period in this data, likely increase the probability of finding 
significant differences, given the dependence in rime in precipitation data. Multivariate analyses of 
crosstabvJated daily precipitation from the Corpus Christi WSO AP gauge express differences in the 
precipitation distribution of the 1950s drought years versus distributions from the other study periods of the 
60-year record under consideration. The second study period precipitation distribution did not differ from that 
of the third and most recent period. 

Inspection of event frequencies of large precipitation amounts (=> 4 inches per day) revealed 25 events in the 
Corpus Christi WSO AP data. There were 14 years of the record with tropical cyclone occurrences (8-La Niiia, 
3-El Nino, 3-baseline). Ten large events (40 percent) occurred in La Niiia years of which seven events (70 
percent) were during five tropical cyclone years. Seven large events (28 percent) occurred in El Niiio years but 
none in tropical cyclone years. Eight events (32 percent) occurred in baseline years of which three events (38 
percent) were in tropical cyclone years. Seven of 21 years with large events involved tropical cyclones. The 
three largest category events were La Nina events in years without tropical cyclones. Two of the three largest 
events occurred during the drought years of the 1950s. These results suggest somewhat over one-third of large 
precipitation events occur during the La Niiia phase and often in years (La Niiia) with tropical cyclones. A 
modest number of large events seem to occur during the El Nifio phase but seldom involving years with 
tropical cyclones. Large events occur in baseline years, some of which may be tropical cyclone years. These 
results are generally consistent with those of the computed precipitation anomalies. An additional multivariate 
comparison of interest that was not conducted is the comparison of distributions between ENSO types and 
baseline years. 

The crosstabulated data revealed 21 (40 percent) of 52 years included large precipitation events. Of the 21 
years, 6 were El Niiio, 8 were La Niiia, and 7 were baseline years. Tropical cyclones occurred in seven high- 
event years: five were La Niiia years and two were baseline years. Annual precipitation exceeded 40 inches in 7 
years of the record. Five years were baseline years, one year was La Niiia, and one year was El Nino and the 
greatest amoimt (48.07 inches) of the record. Two years of the high seven years did not have a high daily- 
amount event. These results express a role by ENSO but also other mechanisms in causing Nueces River 
watershed precipitation. 



yippendixD ♦ D-9 



Table 3: Frequencies are given from cross-tabulation of Corpus Christi WSO AP daily precipitation. 

Column names are coded to indicate the precipitation category. The LT1 code represents precipitation less 
than one inch but greater than zero, and 1T02 represents precipitation greater than one inch and less than 
or equal to 2 inches, etc. 



YEAR 


ZERO 


LT1 


1T02 


2T03 


3T04 


4T05 


5T06 


6T07 


7T08 


1948 


305 


53 


4 


3 

















1949 


281 


75 


6 


3 

















1950 


311 


47 


5 


1 

















1951 


291 


66 


3 


4 





1 











1952 


297 


62 


5 


2 

















1953 


294 


64 


6 











1 








1954 


311 


50 


3 


1 

















1955 


308 


53 


2 


1 














1 


1956 


314 


48 


2 


1 














1 


1957 


295 


60 


8 


2 

















1958 


274 


78 


6 


5 


1 


1 











1959 


278 


76 


7 


2 


1 


1 











1960 


266 


90 


6 


2 


1 





1 








1961 


297 


61 


4 


1 


2 














1962 


301 


62 


2 




















1963 


305 


56 


3 


1 

















1964 


291 


70 


4 





1 














1965 


284 


75 


5 


1 

















1966 


281 


77 


2 


4 


1 














1967 


297 


58 


6 





3 








1 





1968 


271 


85 


3 


6 


1 














1969 


290 


69 


5 


1 

















1970 


288 


67 


6 


2 





1 





1 





1971 


301 


55 


4 


3 


2 














1972 


274 


83 


4 


3 


2 














1973 


268 


83 


10 


3 





1 











1974 


285 


73 


7 




















1975 


284 


73 


7 


1 

















1976 


281 


72 


10 


2 


1 














1977 


286 


73 


4 


1 





1 











1978 


287 


68 


5 


2 


1 


2 











1979 


278 


78 


6 


2 











1 





1980 


298 


59 


6 


1 











2 





1981 


276 


75 


9 


3 


1 


1 











1982 


307 


54 


1 


2 





1 











1983 


287 


65 


7 


6 

















1984 


299 


61 


3 


3 

















1985 


290 


62 


12 











1 








1986 


279 


76 


8 


2 

















1987 


283 


70 


10 


2 

















1988 


300 


62 


2 


2 

















1989 


305 


55 


5 




















1990 


286 


74 


4 


1 

















1991 


259 


94 


7 


3 





1 





1 





1992 


269 


84 


8 


4 


1 














1993 


292 


63 


3 


3 


4 














1994 


288 


66 


5 


5 








1 








1995 


288 


68 


5 


3 














1 


1996 


311 


50 


4 


1 

















1997 


268 


86 


6 


3 


2 














1998 


287 


69 


7 


1 


1 














1999 


300 


56 


5 


3 





1 












D-10 4* Recent Trends in Precipitation Occurring on the Nueces River Watershed of South Texas 



SUMMARY 

Precipitation from the Nueces River watershed was analyzed for recent trends. Of particular interest were 
comparisons of precipitation across the three periods, 1940-57, 1958-81, and 1982-99, selected for study in the 
potential long-term effects of the Rincon Project (Irlbeck and Ward 2000). Tropical cyclone occurrences in the 
Adantic basin and possible relationships with ENSO and climate variables were investigated. An analysis was 
conducted of Nueces River watershed precipitation anomalies stratified by ENSO type, to assess whether 
ENSO may affect watershed precipitation. A multivariate comparison was conducted of daily precipitation 
sampling distributions for the Rincon three study periods using data from the Corpus Christi WSO AP gauge. 
Differences could point to more potential inundations of the Nueces delta marsh, despite possible similar 
annual precipitation amounts. 

Precipitation analyses indicated highest amounts occiorred along the Nueces Bay coastline of the Rincon Project 
study area. Least amounts occurred in the west-central area of the watershed. Precipitation from three gauges 
located across the watershed displayed the same general pattern across the three study periods: least annual 
amounts during the 1950s' drought, highest amounts during 1958-81 (second study period), and intermediate 
amounts during 1982-99. Using a base period that included the 1950s, annual precipitation portrayed an 
increasing trend. However, a broader time view using a base period that consisted of pre-1950s data produced 
no particular trend in Nueces River watershed precipitation. 

The survey of studies of ENSO and some climate variables, and their possible relationships to tropical cyclone 
activity in the Adantic basin jrielded results of analyses of interannual trends and multi-decadal variability. The 
studies found that only weak Unear trends can be ascribed to the hurricane activity and that multi-decadal 
variation is the stronger characteristic present (in particular; Gray, 1979, 1984; Gray et al, 1993; Landsea, 1993; 
Landsea et al, 1992, 1999). Various environmental factors including are analyzed for interannual links to the 
Adantic hurricane activity. Environmental variables showing significant, concurrent relationships to the 
firequency, intensity and duration of Adantic hurricanes include the stratospheric Quasi-Biennial Oscillation, 
Adantic sea surface temperatures, 200mb zonal winds, Caribbean sea level pressures, African West Sahel 
rainfall, and ENSO. ENSO was linked to changes in tropical cyclone-caused damages. More damage occurred 
during La Nina years followed by baseline years and El Nino years. 

ENSO related precipitation anomalies in the Nueces River watershed were investigated using monthly data 
stratified by ENSO phase to develop proportion of months of record with precipitation that equaled or 
exceeded a benchmark determined from the baseline years. Analyses were separately conducted for three 
gauges of the Nueces River watershed. Results were variable and did not definitively point to either phase 
expressing a consistent anomaly sense (positive or negative). Suggestions in the results include that two of 
three gauges yielded more positive anomalies in La Nina months (eight tropical cyclones) than was obtained for 
El Nino months (four tropical cyclones). Four of six anomalies were negative. Overall, the results suggest that 
high variability in the watershed precipitation can mask whatever ENSO effects occur. Perhaps data for more 
watershed gauges would be more revealing. 

The sampling distribution of Corpus Christi WSO AP daily precipitation was investigated per each Rincon 
Project study period. While annual precipitation changed Utde after the 1 950s drought, possibly the distribution 
of daily amounts differed from one Rincon study period to another. Differing distribution could favor more 
marsh intmdations in some study period. Multivariate analysis was used to compare the sampling distributions 
of the three study periods. Exploratory analyses revealed that the precipitation distribution of the first study 
period that included the 1950s drought differed from each of the other two study periods. The second period 
data did not differ from the third period data, suggesting that neither period would on average be dominant in 
potential natural inundations of the Nueces delta marsh. 

The crosstabulated Corpus Christi WSO AP data revealed 21 (40 percent) of 52 years (since 1948) with large 
precipitation events (=> 4 inches per day): 6 were El Nino years, 8 were La Nina years and 7 were baseline 
years. Tropical cj'clones occurred in seven high-event years: five were La Niiia years and two were baseline 
years. Annual precipitation exceeded 40 inches in 7 years of which 5 years were baseline years, 1 year was La 
Nina, and 1 year was El Niiio and the greatest amount (48.07 inches) of the record. Of the seven high-annual 
years, two years did not have a high daily-amount event. 



Appendix D ♦ D-11 



This study revealed that the most prominent feature of Nueces River watershed precipitation since 1940 was 
the 1 950s drought. No particular precipitation trend was apparent unless the basis of comparison is largely the 
period of drought years. While on average, the La Nina ENSO phase leads to more tropical cyclones in the 
western Gulf of Mexico, no consistent anomaly across the Nueces River watershed was revealed. This result is 
important because the Adantic basin may be returning to a more active tropical cyclone period possibly leading 
to more related storms impacting the Nueces delta. 

ACKNOWLEDGMENTS 

This study was conducted uith funding from the Rincon Project allocation. Suggestions and comments from 
Mike Irlbeck of Reclamation were beneficial and gready appreciated. Editorial reviews were conducted by Mike 
Irlbeck and Gray Harris of Reclamation. 

LITERATURE CITED 

Asquith, W. H., J. G. Mosier, and P. W. Bush, 1997. Status, Trends, and Changes in Freshwater Inflows to Bay 

Systems in the Corpus Christie Bay National Estuary Program Study Area. CCBNEP-17, Coastal 

Bend Bays and Estuanes Program, Corpus Christi, TX, 
Cleveland, W. S. 1979. Robust locally weighted regression and smoothing scatterplots. J. Amer. Stat. Assn., 74, 

829-836. 
Cleveland, W. S., 1981. LOWESS: a program for smoothing scatterplots by robust locally weighted regression. 

The American Statistician, 35, 54. 
Gershunov, A., and T. Bamett, 1998. ENSO influence on intraseasonal extreme rainfall and temperature 

frequencies in the contiguous United States: Observations and model results. /. Climate, 11, 1575-1586. 
Gray, W. M., 1968. Global view of the origins of tropical disturbances and storms. Mon. Wea. Rff., %, 669-700. 
Gray, W. M., 1979. Hurricanes: Their formation, structure and likely role in the tropical circulation. 

Meteorology over the tropical oceans. D. B. Shaw (Ed.), Roy. Meteor. Soc, James Glaisher House, 

Grenville Place, Bracknell, Berkshire, RG12 IBX, 155-218. 
Gray, W. M., 1984. Adantic seasonal hurricane frequenc)': Part I: El Nino and 30 mb quasi-biennial oscillation 

influences. Mon. Wea. ^v., 112, 1649-1668. 
Gray, W. M., C. W. Landsea, P. W. Mielke,Jr., and K. J. Berry, 1993. Predicting Adantic basin seasonal tropical 

cyclone activity by 1 August. Weather Forecasting 8, 73-86. 
Helsel, D. R., and R. M. Hirsch, 1992. Statistical methods in water resources. New York: Elsevier. 522 p. 
Hollander, M., and D. A. Wolfe, 1973. Nonparametric statistical methods. New York: John Wiley. 503 p. 
Irlbeck, MJ. and G.H. Ward. 2000. j\nalysis of the historic flow regime of the Nueces River into the upper 

Nueces Delta, and of the potential restoration value of the Rincon Bayou Demonstration Project. In 

Concluding Report: Rincon Bayou Demonstration Project, Appendix I-C. United States Department 

of the Interior, Bureau of Reclamation, Austin, Texas. 
Knaff, J. A., 1997. ImpUcations of summertime sea level pressure anomahes in the tropical Atlantic region. 

Mon. Wea. Rev., 10, 789-804. 
Landsea, C. W., W. M. Gray, P. W. Mielke, Jr., and K. J. Berry, 1992. Long-term \'anations of Western 

Sahelian Monsoon Rainfall and Intense U.S. Landfalling Hurricanes. J. Climate, 5, 1528-1534. 
Landsea, C. W., 1993. A Climatology of Intense (or Major) Adantic Hurricanes. Mow. Wea. Rev., 121, 1703-1713. 
Landsea, C. W., R. A. Pielke, Jr., A. M. Mestas-Nunez, and J. A. Knaff, 1999. Adantic Basm Humcanes: 

Indices of Climatic Changes. Climatic Change, 42, 89-129. 
Mielke, P. W., K. J. Barry, and E. S. Johnson, 1976. Multi-response permutatation procedures for a priori 

classifications. Commun. Statist. Theor. Meth., A5, 1409-1424. 
Mielke, P. W., K. J. Barry, and J. G. Medina, 1982. Climax I and II: distortion resistant residual analyses. 

/ Appl. Meteor, 21, 788-792. 
Miller, R. G., Jr., 1981. Simultaneous statistical inference. New York: Springer- Verlag. 
Philander, S. G. H., 1989. El Nino, La Niiia, and the Southern Osdllation., Academic Press, New York, 

293 pp. 
Pielke, Jr., R. A., and C. W. Landsea, 1999. La Nina, El Nino, and Atlantic Hurricane Damages in the United 

States. Submitted to Bull Amer. Meteor. Soc. 
Shapiro, L. J., 1989, The relationship of the quasi-biennial oscillation to Atlantic tropical storm activit)'. Mon. 

Wea. Rev., 117,2598-2614. 

D-12 ^* Recent Trends in Precipitation Occurring on the Nueces River Watershed of South Texas 



APPENDIX E 



Utilization of Estuarine Organic 
Matter During Growth and Migration 
by Juvenile Brown Shrimp Penaeus 
aztecus in a South Texas Estuary 



p. Riera Centre de Recherche en Ecologie Marine et Aquaculture de L'Houmeaii, France 

P.A. Montagna University of Texas at Austin, Marine Science Institute 

R. D. Kalke University of Texas at Austin, Marine Science Institute 

P. Richard University of Texas at Austin, Marine Science Institute 

In Press in: Marine Eco/o^-Prvgress Series 

ABSTRACT: The trophic dynamic links of migratory juvenile brown shrimp Penaeus as^tecus were investigated 
along the South Texas coast from the i\ransas Pass to Corpus Christi and Nueces Bay and to the nursery 
ground in the Nueces Delta. Shrimps and their potential food sources were measured for 6'^C and 6'^N ratios 
between December 1995 and July 1996. During this period, shrimp length increased from 10-11 mm when the 
animals entered Corpus Christi Bay as larvae to 80-90 mm when they returned to Mexico Gulf as subadults. 
Brown shrimp exhibited spatial and temporal 6'^C variation (from -25.2 to 12.5%o) indicating a high diversity of 
food sources throughout their migration. From o"C values, the main sources used as food sources by juvenile 
brown shrimp in the Rincon Bayou marsh, were Spartina altemiflora and Spartina spartinae detritus and benthic 
diatoms. o'^C and o'^N values showed that organic matter inputs carried by the river inflow can also contribute 
significandy to the feeding of migratory brown shrimp. In these marsh habitats, shrimp isotopic ratios changed 
rapidly suggesting high tissue turnover rates. The study showed that coastal marshes after restoration through 
the introduction of freshwater inflow may provide feeding habitats favorable for growth and development of 
juvenile brown shrimp. 

KEY WORDS: Penaeus a^eats, food sources, migration, nursery area, 6"C and 6''N 

INTRODUCTION 

Many marine species utilize salt marshes, coastal lagoons and estuaries dixring part of their life cycle (Day et al. 
1989). The brown shrimp, Penaeus aztecus Ives, is widely distributed along the Texas coastline where it is an 
important commercial fishery. Like most penaeids, the life history of brown shrimp is complex (Dall et al. 
1990). In the Gulf of Mexico, after offshore spawning, post-larval brown shrimps are carried by on-shore water 
movement and enter bays and shallow estuarine waters where they generally find productive areas and are 
protected from storms and predators (Day et al. 1989, Fry 1981). Following growth of juvenile shrimp in 
nursery areas there is a subsequent offshore migration of subadults to complete their life cycle. More precisely, 
throughout the Texas bay systems, most of the larval brown shrimp enter marine bays from late winter through 
early spring, spend about 3-4 months in estuarine nursery grounds, and return to the offshore Gulf of Mexico 
in eady summer (Moffett 1970). However, the shrimp fishery is an exploited resource in the Gulf of Mexico 
(Parker et al. 1988). Restoration of coastal marshes, which act as nurseries, could contribute to increased brown 
shrimp populations because the marshes provide habitats for juveniles. Enhancing freshwater inflow may have 
at least two beneficial effects for restoring shrimp habitats: terrestrial inputs may be used as food sources 

Appendix E ♦ E-1 



(Hackney & Haines 1980 Riera & Richard 1996), and freshwater inflow may increase the total primary 
production through nutrient inputs (Nixon et al., 1986). 

The food sources utili2ed by juvenile brown shrimp during migration are more difficult to determine than that 
of adults, which live in deeper offshore environments that are generally phytoplankton-based systems (Frj' & 
Parker 1979). As juvenile penaeids migrate, they occupy different habitats, which correspond to different 
feeding grounds (Dall et al. 1990). Food availabilit}' can differ gready within and between these habitats (Frj^ 
1981), and different food sources may be used preferentially by shnmp. For example, stomach content analyses 
of small juvenile Penaeus semisculatus vittt composed of a large variety of prey including diatoms, meiofauna, 
insect larvae and seagrass (Heales et al 1996). Moreover, Wassenberg & liill (1987) observed large intraspecific 
differences in the food ingested by Penaeus esculentus collected from widely separated areas. Using immunological 
methods, it was found that the Penaeus ai^ecus and Penaeus setiferus have a diverse diet (Hunter & Feller 1987). 

Stable isotope analysis has been used successfully to determine original food sources of marine and estuatine 
invertebrates (Harrigan et al. 1989). Stable isotopes assess food sources assimilated over time (Fry & Sherr 
1984), so they are valuable for feeding studies when material in gut contents are difficult to identify due to 
digestion and trituration. \^ariarion in d -^C values of the migratory brown shrimp along the South Texas coast 
has been investigated previously (Fry & Parker 1979, Fry 1981). These studies have pointed out that seagrass 
meadows of shallow-water habitats were important feeding grounds for migratory juvenile brown shrimp. 
However, littie is known about spatial and temporal variation of food sources encountered by brown shrimp 
throughout a complete migration between oceanic waters and upper estuarine reaches. As suggested by Fry 
(1981), habitats other than seagrass meadows may contribute significandy to the feeding of migrating shrimps. 
Therefore, it is important to know which feeding habitats contribute the most to the growth and development 
of the brown shrimp along the Texas coastline. 

The aim of the present study was to identify the trophic dynamic links of migratory Penaeus a^ecus with food 
sources in various habitats along the South Texas coast. Shrimp migrations were followed from the Aransas 
Pass to Corpus Christi and Nueces Bay and to the ultimate nursery ground in the Nueces Delta. A primary 
objective was to determine if the p rim ary production in a coastal marsh, which is being currendy restored by 
the re -introduction of freshwater inflow, can support the feeding and growth of juvenile brown shrimps. 
Shrimp and potential food sources were determined by stable isotope analyses (6"C and 6'*N). 

Material and Method 
Sampling Area 

The study was carried out in the Nueces Estuary (Fig.l). The estuary consists of the Nueces River, the Rincon 
Bayou marsh and the Rincon Bayou mouth. The Nueces River empties into Nueces Bay, which is connected to 
Corpus Christi Bay, which is connected to the Gulf of Mexico by the Aransas Pass. Historically, the Nueces 
River fed into Rincon Bayou, which is in the center of the Nueces Delta. A dam was created early in the 20* 
century to contain Nueces River water. Recendy, the dam was lowered to allow flood events from Nueces River 
to flow into the Rincon Bayou to restore the Rincon Bayou marsh. Sampling locations into Rincon Bayou 
marsh (Fig.l) were distributed from the freshwater entrance into Rincon Bayou marsh (River site) to the Rincon 
Bayou mouth (Rincon mouth). Sampling stations also included "Up Marsh" and "Down Marsh" within Rincon 
Bayou marsh, and "Aransas Pass" which connected Corpus Christi Bay to the Gulf of Mexico. These sites have 
been considered because, as reported by Moffett (1970), they may lie on the migratory route of brown shrimps 
that enter Corpus Christi Bay through Aransas Pass. 

Collection and Preparation of Samples 

Sampling for organic matter sources and brown shrimp was carried out during from December 1995 to July 
1996. Shrimp were collected by otter trawls or hand-thrown cast nets. They were sorted by hand and kept in 
seawater from the sampling site so that guts would be purged prior to analysis. At the laborator)', shrimp were 
identified to the species levels using a magnifying glass and keys, measured (length) and were frozen (-80°C). 
For stable isotope analyses shrimp were prepared according to Fry & Parker (1979). White muscle tissue was 
dissected from the shrimp abdomen, acidified (10% HCl) to remove any residual carbonates from cuticules, 

E-2 ^ Utili:(ation of Estuarine Organic Matter During Gnwth 
and Migration by Juvenile Brown Shrimp 



tinsed with distilled water, and dried in a oven at 60°C. Then, the white muscle of each individual was ground 
to a fine powder with a mortar and pesde to homogenize the sample. Individuals were used as samples except 
where shrimp were too small to obtain sufficient tissue for accurate 6"C and 6'^N analyses. For larval shrimp, 
6 or 8 individuals (10-11 mm size length) from the same sampling occasion were combined. Some of the brown 
shrimp collected and analysed for 6"C were also measured for 6'^N. Zooplankton was collected with a (176 
|Jm) mesh net in Nueces Bay near Rincon mouth (Fig.l). Samples of zooplankton were acidified (10% HCl) to 
remove any residual carbonates from cuticules, rinsed with distilled water, dried in a oven at 60°C and ground 
to a fine powder with a mortar and pesde to homogenize the sample. 

Corixidae Corix sp and mysids Mysidopsis almyra were sampled at Up Marsh at different occasions, between April 
1995 and February 1996, by using the same procedure as for brown shrimp. Composite samples of corixidae (8 
individuals) and mysids were prepared in the same way as for zooplankton and brown shrimp, respectively. 

For sampHng particulate organic matter (POM), 15 1 of water were collected, then filtered on precombusted 
Whatman GF/F glass fiber filters under moderate vacuum within five hours after collection. Samples were 
acidified (10% HCl) to remove carbonates, dried at 60°C and kept frozen (-80°C) vintil analysis. For sedimented 
organic matter (SOM) analysis, sediment samples were taken in the Nueces River at "River site" and in the 
Rincon Bayou marsh at "Up Marsh" and "Down Marsh" by scraping the upper 1 cm of mud. At the laboratory, 
samples were homogeneized, dried at 60°C, ground using a mortar and a pesde, and then acidified (10% HCl) 
to remove any inorganic carbon. These samples were not rinsed to prevent any loss of dissolved organics. They 
were dried overnight at 50°C under a slight vacuum to evaporate the acid. Once dried, the sediment was mixed 
with Milli-Q water, freeze-dried, ground again to a fine powder and kept frozen (-80°C) until analysis. 

At "Rincon mouth", leaves and twigs of the two dominant marine phanerogames, Spartina altemlflora and 
Salicomia sp, were collected. For samples of terrestrial organic matter, leaves of the dominant vascular plants, 
namely Salix sp, Fraxinus sp and the switchgtass Panicum virgatum, were collected along the Nueces River up 
"River site". Most samples within Rincon Bayou marsh were the Gulf Cord Grass Spartina spartinae which 
dominates along the Rincon Bayou channel. These plant samples were cleaned of epibionts, and prepared 
similarly to shrimp muscle tissue. Blue green algal mats were collected in the Rincon Bayou channel and 
acidified (10% HCl), rinsed with distilled water, freeze-dried, and firozen until analysis. Benthic diatoms were 
also sampled from muddy sediments near the Rincon Bayou channel, and separated using a procedure from 
Couch (1989) as modified by Riera & Richard (1996). Briefly, the surficial sediments with dense microalgal mats 
was scraped and brought into the laboratory where it was spread on flat trays to a depth of about 1 cm. A 
nylon screen (63 p.m mesh) was laid upon the sediment surface and covered with a 4 to 5 mm layer of 
combusted silica powder (60-210 p.m). After 12 hours, the top 2 mm of the sihca powder was gendy scraped 
and then filtered on previously combusted glass fiber filters, acidified (10% HCl), rinsed with distilled water, 
freeze-dried, and frozen (-80''C). 

Stable Isotope Analysis 

Samples for isotope analyses were combusted at 900°C using CuO as an oxydant in evacuated quartz tubes 
(Stump & Frazer 1973). Samples for isotope analyses were prepared as in Boutton (1991). Before the 
purification of CO^, N^ was trapped on silica gel granules in a stopcock sample ampule and analyzed immediady 
after CO, collection (Mariotti 1982). The carbon and nitrogen isotope ratios were measured using a Sigma 200 
(CJS Sciences) double inlet, triple collector isotope ratio mass spectrometer. Data are expressed in the standard 
d unit notation where 6 X = [(R,^piyR„fe^cc)-l] x 10\ widi R = '^C/''C for carbon and '*N/"N for nitrogen, 
and reported relative to the Pee Dee Belemnite standard (PDB) for carbon and to air N^ for nitrogen. The 
typical precision of the overall procedure (i.e., preparation plus analysis) was ± 0.1 %o for carbon and ± 0.2 %o 
for nitrogen. 

Results 
6"C AND 6'^15N OF POM, SOM, Sources and Invertebrates 

O'^C and o'*N of POM, SOM, plant sources and invertebrates are presented in Table 1. There was a gradient in 
carbon isotope values from the sea to the river. At Aransas Pass, POM 6"C values were 6'^befween -24.8%o 

Appendix E ♦ E-3 



and -22.0%o while at the Nueces River site, POM 6"C ranged from -28.8 to -26.3 %o. POM 6"C at Rincon 
Bayou mouth ranged from -27.2%o to -20.8%o. ^X'ithin Rincon Bayou marsh, POM exhibited a large range for 
6"C, from -26.3%o to -17.4%o at Down Marsh, and from -24.1%o to -17.r/oo at Up Marsh. Corresponding 6'*N 
values for POM ranged from 3 to 11 %o at Aransas Pass, from 8.8 to 10.8 %o at Nueces River and from 9.2 to 
9.4 %o at Rincon Bayou mouth. Within Rincon Bayou marsh, POM 6'^N ranged from 2.6 to 9.3 %o. 6"C for 
SOM (i.e., including benthic algae) ranged from -24.0 to -22.2%o at Nueces River and from -21.8 to -20.2 %o in 
Rincon Bayou marsh. SOM 6'^N values were between 7.1 to 8.2 %o in Rincon Bayou marsh. At Rincon Bayou 
mouth, o'^C values ranged from -27.8 to -26.3 %o for SaUcomia sp and from -16.2 to -13.7 %o for Spartina 
altemifhra, tj-pical of 6'^C for C3 and C4 plants, respectively (Fry & Sherr 1984, Currin et al. 1995) and close to 
values previously observed for Spartina sp and Salicomia sp by Creach (1997). Within Rincon Bayou marsh, 6"C 
for Spartina spartinae ranged from -16.8 to -14.5 %o. Benthic diatoms inhabiting muddy sediments near the 
Rincon Bayou channel had 6'^C values from -18.5 to -16.3 %o. Blue green algae had 6'^C from -15.7 to -15.9 
%o typical of "C-enriched surfidal blue-green algal mats from Texas (Calder & Parker 1973). Blue green algae 
were the most '*N-depleted primary producers with 6'^N from -0.7 to 1.7 %o. 6"C values for leaves of the 
most common terrestrial vegetation along Nueces River, from -30.3 to -27.6 %o, for Fraxinus sp and Sa&x sp, 
were typical of terrestrial C3 plants (Degens 1969). 6"C oi Panicum virgatum ranged from -15.9 to -14.6 %o 
indicating a C4 photosynthetic pathway (Fr)' & Sherr 1984). At Rincon Bayou mouth zooplankton was "C- 
depleted (from -26.3 to -25.6 %o) and '^N-enriched (1 1.7 %o). Within Rincon Bayou marsh Corix sp and 
Mysidopsis almyra showed 6'^C from -20.8 to -16.2 %o and from -21.6 to -20.5 %o, respectively. Corresponding 
6'*N values were -0.2%o for Corix sp and from 10.5 to 10.8 %o for Mysidopsis almyra. 

Size Length, 6"C and S'^N of Penaeus aztecus 

Early in the study, only small larval shrimp were found in the Aransas Pass during their migration into the bay 
(Fig. 2). Shrimp length increased from 10-11 mm (larvae) injanviaiy 1996 to 80-90 mm (subadults) in July 1996 
when they migrated seaward, out of the Pass. Shrimp size increased from about 40 mm to 60 mm in the nursery 
habitat (Fig. 2). Isotope values of 6"C and 6'*N for Penaeus a^ecus also differed between i\ransas Pass and 
Nueces River and within sites over the period Januar)' to July 1996 (Table 2). >\lthough the sampling procedure 
was similar at each sampling occasion, the number of shrimp collected was unequal at the different sites and 
dates, owing to abundance fluctuations throughout the migration. In particular, on the different sampUng dates, 
brown shrimps were not found at every site (Table 2) confirming the migratory pattern of the juvenile brown 
shrimp previously observed along the South Texas coast (Moffet 1970, Fry 1981). However, the sampling 
procedure did not allow an accurate quantitative evaluation of shrimp abundance at the different sites over the 
sampUng period. 

Shrimp feeding habitats and assimilated carbon and nitrogen were different in the animals at various stages of 
the migratory life cycle. At i\ransas Pass, 6'^C values for brown shrimps varied between -21.7 and -20.7 %o for 
larval stages (10-11 mm) in winter and from -16.4 to -12.5 %o for sub-adults (80-90 mm) in summer. 6''C 
values for juvenile shrimps ranged from -20.6 to -14.8 %o at Rincon Bayou mouth, from -21.3 to -14.5 %o at 
Down Marsh and from -18.4 to -15.3 %o at Up Marsh. At the Nueces River site, brown shrimp were collected 
between mid-May and mid-lune and their 6'^C ranged from -25.2 to -20.4 %o. 6"C for Penaeus a^ecus^e.r& 
significantly different among the sampling sites (Kruskal-WalHs test=35.3, df=4, p<0,001). At Aransas Pass, 
o'^N values were between 5.5 and 7.7 %o for larval shrimp and between 3.3 and 9.7 %o for sub-adults. o'*N 
ranged from 4.5 to 13.1 %o at Rincon Bayou mouth, from 1.0 to 9.9 %o at Down Marsh and from 1.2 to 5.1 %o 
at Up Marsh. At the Nueces River site, 6"N values ranged between 6.6 and 13.9 %o. 6'^N values were 
significandy different among the sampling sites either (Kruskal-Wallis test=36.8, df=4, p<0,001). 

DISCUSSION 

6"C Variations of Penaeus aztecus Throughout the Migration 

spatial and temporal O'^C of migrating brown shrimp exhibited variation throughout migration (Fig.3) 
confirming the migratory pattern of juvenile brown shrimp in Texas bays previously observed (Moffet 1970, 
Ft)' 1981). The range of 6''C values for Peneaus amicus (from -25.2 to -12.5 %o) is similar to the range of o"C 
observed for the main sources of organic matter (from -30.3 to -13.7 %o). This observation is consistent with 

E-4 ^ Utilis^ation ofEstuarine Organic Matter During Growth 
and Migration by juvenile Broom Shrimp 



the hypothesis that a high diversity of food sources is used by brown shrimp throughout the migration. 
However, during each sampling date from January 25 to July 29 (Table 2, Fig.3), there was little variation in 
shrimp 6'^C values (sd < 2 %o), indicating that the shrimps had a similar diet at specific sites and times (de Niro 
& Epstein 1978). Wide ranges of 6"C for invertebrates have been previously observed along salinity gradients 
(Incze et al. 1982, Hughes & Sherr 1983). The oyster Crassostna gigas exhibited significandy different 6"C 
variations between sites along an estuarine gradient, reflecting the preferential utilization of different food 
sources, namely, terrestrial detritus, benthic diatoms and marine phytoplankton (Riera & Richard 1996). The 
present study suggests that individual 6''C variation of a migrating species (Penaeus a^ecus) along an estuarine 
gradient can be as large as inter-individual o"C variation of a sedentary species {Crassostna gigas). 

Contribution of salt marsh sources to brown shrimp diet 

Salt marsh habitats appear to be important food sources for young brown shrimp. When entering Corpus 
Christi Bay through Aransas Pass, brown shrimp larvae had typically O C values (-21.7 to -20.7 %o) 
characteristic of animals feeding primarly on an oceanic planktonic food soiurce (Fry & Parker 1979, Incze et al. 
1982). However, as they entered the mouth of Rincon Bayou through the Spartina altemiflora and Salicomia sp 
marsh, brown shrimp became more "C -enriched (-19.4 to -16.8 %o) indicating a significant contribution of a 
"C-enriched source to shrimp diet. At Rincon mouth, suspended POM, SaUcomia sp and zooplankton were too 
"C-depleted (-27.3 to -24.2 %o) to explain the enrichment in "C of migratory brown shrimp (Table 1, Fig.3). 
Moreover, assuming phytoplankton is the main food source for zooplankton, and that the trophic O C 
enrichment is about 1 %o per trophic level (de Niro & Epstein 1978), phytoplankton O C should be about -27 
%o, which is too negative to be a major contribution to brown shrimp diet. The "C-enrichment of brown 
shrimp at the mouth of Rincon Bayou can be explained by a significant contribution of carbon derived from 
Spartina altemiflora detritus (-16 to -14.5 %o). Consistent with these results, plant detritus derived &om Zostera sp, 
Vhragmites sp or Spartina sp were found in gut contents of post-larval penaeids indicating that marsh detritus may 
be a food source for these shrimps when they occupy that habitat (see Dall et al., 1990 for a review). Likewise, 
mangrove detritus has been shown to contribute to the diet of juvenile Penaeus merguiensis inhabiting tidal creeks 
in Peninsular Malaysia (Newell et al. 1995). In contrast, from feeding experiments Gleason & Wellington (1988) 
reported that Spartina altemiflora detritus and its epiphytes contributed only a small part of Penaeus a^ecus 
assimilated carbon. Finally, Dall et al (1990) concluded that plant detritus itself is not a major food source for 
prawns. 

The results of the present study indicated that detritus derived from Spartina altemiflora can be an important 
carbon source to juvenile brown shrimp. However, Penaeus a^ecus may assimilate carbon that is ultimately 
derived from Spartina via several routes other than direct feeding on plants detritus. It is possible that Penaeus 
a^^ecus may obtain part of its carbon derived from Spartina altemiflora detritus through microbial mediation. For 
example, in '''C labelling experiments, the grass shrimp Palaemonetes puffo could assimilate carbon from detrital 
Spartina altemiflora with 38,4 % efficiency via bacterial mediation between non-living organic detritus and shrimp 
(Crosby 1985). In fact, bacteria associated with debris of refractory plant material can facilitate the carbon 
transfer from plant sources to bivalves (Langdon & Newell 1990, Crosby et al. 1990). 

As brown shrimp occupied the down and up marsh, they remained "C-enached (-19 to -17 %o), indicating a 
persistence in their utilisation of a relatively heavy ''C source (Fig.3). Although there is no Spartina altemiflora 
within the Rincon Bayou marsh, these 6"C values may be explained by utilization of benthic diatoms, blue 
green algae and/or detritus derived from Spartina spartinae as carbon source (Fig.3). However, the respective 
contributions of these '^C-enriched sources to brown shrimp feeding cannot be established from 6'^C values 
alone. It is known that benthic microalgae from mudflats represent one of the dominant food source for 
juvenile penaeids within tidal creeks in Peninsular Malaysia (Newell et al. 1995). Also, a positive growth rate of 
postlarval brown shrimp can be supported over 16 days by feeding only on the planktonic diatom, Skeletonema 
costatum (Gleason & Zimmerman 1984). Although living microalgae can be more readily used than detritus of 
vascular plants, as shown for marine bivalves (Bayne et al 1 987, Crosby et al 1 989), a significant contribution of 
detritus derived from Spartina spartinae to shrimp diet may also accoimt for the observed carbon isotope values. 
These results are in accordance with recent isotopic data of Deegan and Garritt (1997) showing a preferential 
utilisation of local sources organic matter in coastal marsh areas by invertebrates. 



Appendix E ♦ E-5 



Contribution of Riverine Inputs to Brown Shrimp Diet 

6"C values also changed as shrimp moved near and into areas that are subject to freshwater inundation. In May 
and June 1996, juvenile brown shrimp were collected at the Nueces River site up to Rincon Bayou marsh 
(Fig.l). Other shrimp, (e.g. Penaeus merguiensis (Staples 1980) and Penaeus setiferus (Dall et al. 1990)), have also 
been observed in lower salinities environments far upstream from the river mouth. As juvenile brown shrimp 
occupied Nueces River, their O'^C became significandy more negative (-24 to -21 %o) compared with o"C 
measured within Rincon Bayou marsh (Fig.3). This depletion in '^C indicates that a significant part of brown 
shrimp carbon at Nueces River is derived from terrestrial detritus and/or riverine phytoplankton carried by 
freshwater inflow, because Rincon Bayou marsh lacks a "C-depleted source (Fig.3). Unfortunately, 6"C for 
riverine phj'toplankton could not be estimated in the present study. However, previous results showed 
phytoplankton 6"C values of -44 to -47 %o in rivers (Rau 1978) and of -40 %o (Hedges et al. 1986) in a lake. 
Similarly, 6"C of freshwater phytoplankton in the Charente River (France) ranged between -41.8 and -31.2 %o 
(Riera & Richard 1996). Considering this higher depletion in "C for freshwater phytoplankton compared with 
terrestrial C3 plants, a primar)' contribution of riverine phytoplankton to the diet of brown shrimp is tinlikely, 
but cannot be excluded from these results. 

In fact, because the metabolic '^C-enrichment dviring assimilation is close to 1 %o (De Niro & Epstein 1978, 
Rau et al. 1983), shrimp 6'^C values at Nueces River are consistent with a significant utilisation of terrestrial 
C3 plants (-29 to -28 %o) as food source. Moreover, the similarity of 6"C values of terrestrial C3 plants {Salix 
sp, Fraxinus sp) and POM in the Nueces River indicates that detritus firom C3 plants contribute predominandy 
to river POM. Therefore, the resiJts of this study can explain the depletion in '^C for brown shrimp observed 
in lower salinity bays that are flushed by freshwater inflow or are most influenced by river inputs along the 
South Texas coast (Fry 1981). This result is consistent with the hypothesis that terrestrial organic inputs could 
be incorporated into estuarine food weebs (Hackney & Haines 1980, Incze et aL 1982). In fact, previous results 
showed the significant contribution of terrestrial detritus derived from C3 plants to the diet of oysters 
{Crassostrea gigas) in the upper reaches of the Charente Estuary (Riera & Richard 1996), and in the middle 
estuarine reaches as a high river discharge period occurred (Riera & Richard 1997). In addition, from o"C 
analyses, Stephenson & Lyon (1982) reported that the bivalve Chione /tofrA^w^; inhabiting the Avon-Heathcote 
Estuary (New Zealand) could utilise carbon of terrestrial origin depending upon its position in the estuary' and 
local hydrolog)'. Freshwater inputs can be an important source of nutrition for juvenile brown shrimp in 
habitats lacking salt marsh plants and benthic diatoms. Therefore, during periods of high river discharge, 
riverine inputs may support a substantial part of the food webs in South Texas bays and elsewhere as well. 

Offshore Migration of Subadult Penaeus aztecus 

Subadult Penaeus a:(tecus that are migrating offshore have different diets than the subadults found in marshes. Aa 
enrichment in "C for Penaeus a^ecm (-13.8 ±1.5 %o) was observed at a\ransas Pass at the end of July 1996 
(Fig.3). Brown shrimp in Aransas Pass were likely returning towards the nearshore Gulf of Mexico because they 
were sub-adidt size and offshore migration occurs in summer in Texas bays (Moffet 1970). Similar 6"C values, 
between -12.6 and -14.6 %o, were also obser\'ed by Fr}' & Parker (1979) for brown shrimp collected offshore in 
the Gulf of Mexico, but more negative 6''C were observed for other shrimps {Penaeus setiferus) from the same 
area. This enrichment in "C in shrimp tissues is not Ukely to be a result of a metabolic effect as shrimp grow 
due to a variation of carbon fractionation. In fact, the inter-individual 0"C variability among animals having a 
similar food source does not usually exeed 2 %o for fishes and invertebrates, these differences being attributed 
to size, age or sex (Fry & Parker 1979, Hughes & Sherr 1983). The 8 %o mean enrichment in "C in shrimp 
tissues (Fig.3) between the marsh and pass locations in spring and summer may have other interpretations. 

At Aransas Pass, phj'toplankton was likely to be the main organic matter source for brown shrimps because 
there are no seagrass meadows. However, considering the 6'^C of -22.7 %o for marine phytoplankton in the 
Northern Gulf of Mexico given by Thayer et al. (1983), the enrichment in ''C due to metabolic fractionation 
between phytoplankton and brown shrimps would be much more than 1 %o. Most likely, less negative 0"C for 
brown shrimp, as they enter the offshore area, may be explained by a progressive enrichment in "C as shrimps 
returned from Nueces River environments towards offshore waters (Fig.3), as su^ested by Fry & Parker 
(1979). In fact, the shrimps collected in late July, at the end of the migration, exhibited 6"C typical of the 
feeding habitats recendy encountered where they used "C -enriched food sources, e.g., marsh grass or 

E-6 ♦♦♦ Utilisation of Estuarine Organic Matter During Growth 
and Migration by Juvenile Brown Shrimp 



seagrasses. ParticvJaily, within Corpus Christi and Redfish Bays, seagtasses have the highest 6"C values (-3 to - 
13 %o) in the ecosystem as reported by Fry and Parker (1979). High 6"C of -10 %o were also observed recendy 
for seagrasses of Laguna Madre in south Texas (Street et al. 1997). Additionally, macroalgae is known to occur 
in bay bottoms and along the jetties at Aransas Pass, but we have not sampled this source. As they returned 
towards marine waters through Rincon Bayou mouth shrimp may also increase their feeding on Spartina detritus 
direcdy or through predation on detritivores. Therefore, as subadult brown shrimp feed offshore, their 6''C 
should progressively converge on the 6"C value characteristic of offshore environment, close to -18 %o (Fry 
1981). 

Temporal 6"C Variation: Importance for Tissue Turnover 

Tissue tiunover rates are important to know to interpret temporal 6"C variation. There was about a 3.5 %o 
decrease in 6"C values from the up marsh to the Nueces River site (Fig.3) indicating a high tissue turnover rate 
for brown shrimp as they migrate. This isotopic 6"C variation occurred over a distance of less than 5 km and 
within a period of 9 weeks. From one feeding habitat to another the "old carbon" of shrimp tissue is 
progressively diluted due to 1) growth of new tissue using "new carbon" and 2) metabolic loss due to tissue turn 
over (Anderson et al. 1987). Therefore, after a variation in food, shrimp 6"C will change isotopically as rapidly 
as tissue turnover rate will allow (Fry 1982). In the present study, the 6"C decrease of migratory brown shrimp 
is consistent with a high tissue turnover rate, which support the hypothesis of a high growth rate in the nursery 
habitat. This suggestion is consistent with previous results based on experimental observations showing that 
posdarval shrimp can increase in weight by a factor of 4 within a week or less at 25°C (Zein-Eldin & Aldrich 
1965). A 14%o variation for o"C of posdarval brown shrimp has been observed after a 3.9 fold increase in 
weight after a change in food source, indicating a high tissue turnover rate (Fry & Arnold 1982). From feeding 
experiments Gleason (1 986) showed that the half-life of the initial tissue carbon of Penaeus a^ecus fed with plant 
and animal diets was reached before the first doubling of weight. Finally, juvenile shrimp (initially 6"C:-18.6 
%o) in an experimental feeding pond with feed at 6'^C: -22.9 %o for 8 weeks attained an eqtiilibrium 6"C at - 
21.3 %o after 3 weeks and an increase in weight of 300 % (Parker et al. 1988). High tissue turnover rate for 
young shrimp can be related with behaviour and feeding activity. In fact, small juveniles of Penaeus semisculatus 
were active and fed both day and night and are thought to feed continuously and to digest most of their food 
within only one hour (Heales et al 1996). It is likely that the variation in carbon isotope values of brown shrimp 
in the Rincon Bayou Marsh are a result of changed food sources and rapid growth in this nursery habitat. 

Food Sources Determination from 6'*N Values 

Although only part of the shrimp collected were measured for 6'*N, a dual isotope approach is useful for 
identifying food sources. In up and down Rincon Bayou marsh, O'^N for brown shrimp showed a high 
variability both between sampling periods and between individuals (Table 2). This is consistent with a higher 
range in o'^N values for sources in this habitat (i.e., from -0.7 to 7.6%o) and suggest a high diversity of nitrogen 
sources for shrimps (i.e., Spartina spartinea, benthic diatoms, blue green algae). Lower o'^Nvalues that were 
observed for some shrimps in Rincon Bayou marsh could be explained by a depletion in '*N during nitrogen 
assimilation (Mako et al 1982). In fact, these authors observed a mean 6'^N fractionation of -0.3%o for the 
amphipod Amphithoe vaUda fed with fresh and detrital algae. However, this hypothesis is unlikely for Penaeus 
species because feeding experiments demonstrated a trophic enrichment in '^N of 2.4%o for Penaeus vannamei 
(Parker et al. 1988). These lower 6'*N values for brown shrimp may partly result from a preferential 
assimilation of specific chemical components of plant tissues (Mako et al 1982) and/or by the utilisation of '^N- 
depleted blue green algae (Table 1). Therefore, in the present study, 6'*N was not as valuable as 6"C to 
characterize food sources of brown shrimp. This result is consistent with the suggestion of Fry & Sherr (1984) 
that o'^N is not as discriminating as 6"C for food sources determination in coastal ecosystems. However, at 
the Nueces River mean 6'^N for shrimp (i.e., 8.2, 10.1 and 1 1.7 %o) are consistent with a significant utilization 
of the terrestrial C3 plants Fraxinus sp (i.e., 6.6 %o) and/or Salix sp (8.3 ±1.1 %o) when taking into account the 
mean trophic enrichment for 6'^N (i.e., 3.5 %o given by Minagawa & Wada 1984). In this riverine habitat, where 
terrestrially-derived organic matter dominated, 6'*N values confirmed 6"C results. 



Appendix E ♦ E-7 



Importance of Trophic Mediation Through Predation 

Predation on animals can also be important for brown shrimp. Like many Penaeus species, Penaeus a^ecus can 
feed on different prey taxa including oligochaetes, polychaetes, crustaceans, mysids, moUusks and meiofauna 
(Hunter & Feller 1987, Dall et al. 1990). In fact, the utilization of detritus derived from terrestrial and marsh 
vascular plants could occur indirecdy through infaunal prey or through bacterial mediation (Gleason & 
Zimmerman 1984). For example, juvenile Penaeus merguiensis tany use mangrove leaf detritus as a food source 
indirecdy through predation on small detritivorous invertebrates (Newell et al. 1995). Detritus from Spartina 
altemiflora salt marshes may be predonunant in the diet of meiofauna when an accumulation of detrital Spartina 
is associated with the development of an important microbial biomass (Couch 1989). Then, meiofauna may also 
have a role in the trophic mediation between plant detritus and brown shrimp in these habitats. Likewise, 
benthic diatoms can be an indirect food source for juvenile shrimp through direct grazing or through 
intermediate infaunal prey (Stoner & Zimmerman 1988). In the present study, Corixidae {Corix sp) and mysids 
{Leptomis sp and Mysidopsis almyra), which were collected frequendy at the Nueces River and the Rincon Bayou 
marsh, could be part of the diet oi Penaeus a^ecus. 6"C values (mean: -17.9%o) and 6'*N (-0.2%o) of Conxsp 
may explain partly the enrichment in "C and the depletion in '^N observed for individuals of Peneaus a^ecus in 
Rincon Bayou marsh (Table 2). In contrast, isotopic values measured for both for zooplankton near Rincon 
Bayou mouth and Mysidopsis almyra in Rincon Bayou marsh were too "C-depIeted and '*N -depleted to 
contribute significandy to the feeding of brown shrimp in these two habitats. 



Variation in Trophic Level of Brown Shrimp 

Brown shrimp may vary their food sources and feed at different trophic levels during their migratory life cycle. 
Variations in brown shrimp feeding can be direcdy related with habitats. However, the habitat variations may be 
associated with a variation in the trophic level of shrimp. Variations in the diet of Penaeus sp with size and age 
were partly attributed to the variation from a herbivorous to a carnivorous feeding mode as shnmp grow 
(Chong & Sasekumar 1981). Therefore, shrimp 6'^N should become more positive as size increases. This effect 
should be more obvious in animal tissue with 6"N than 6''C values, due to a higher 6'^N fractionation during 
assimilation. The relationship between 6'''N and shrimp length varies in each feeding habitat occupied by 
migratory brown shrimp, because nitrogen isotopic values at the base of the food chain can vary among the 
different habitats (Fig.4). At Aransas Pass, this effect should have been especially clear, because brown shrimp 
size range was largest there. However, there was no significant Pearson correlation between 6'*N and shrimp 
size at Aransas Pass (r=-0.22, p>0.5), at Rincon Bayou mouth (r=0.12, p>0.5), at Rincon Bayou marsh 
(r=0.18, p>0.1) and at Nueces River (r=0.35, p>0.1). These results mdicate that differences in food sources of 
Penaeus a^ecus thoughout its growth and migration are not associated with an increase in trophic level with size 
but mosdy related to feeding habitats, as indicated by 6"C. 

In addition, large ranges of o'^N observed for shrimps within the different habitats (Fig.4) suggest that shrimps 
may use the different sources both direcdy and indirecdy through predation. This hypothesis could explain a 
higher variability for shrimp 6'^N values compared to corresponding 6"C (Table 2) due to the higher trophic 
enrichment during nitrogen assimilation. Therefore, the results of the present study are consistent uith an 
omnivore feeding mode for migratory juvenile brown shnmp. This is concordant with gut content analyses, 
which indicate that animal prey is part of the food ingested by Penaeus a^ecus, but that there is no variation in 
dietary breadth and prey preference as shrimp grow (Hunter & Feller 1987). 

In conclusion, 0"C values suggest that the main food sources for juvenile brown shrimp {Penaeus a^ecus) in the 
Rincon Bayou marsh are Spartina detritus, benthic diatoms and blue green algae. Tissue turnover rates in these 
marsh habitats are apparendy high, because shrimp isotopic signatures change rapidly. Moreover, O'^C and 
6'^N results show that terrestrial inputs carried by the freshwater inflow can contribute significandy to the diet 
of juvenile brown shrimp in the Nueces River. Finally, these results show that the restoration of coastal 
marshes through the introduction of freshwater inflow can provide nursery areas favorable for feeding and 
growth of juvenile brown shrimp. 



\Jtilit(ation ofEstuarine Organic Matter During Growth 
and Migration hy Juvenile Brown Shrimp 



ACKNOWLEDGMENTS 

Funding for this project was provided in part by the U.S. Bureau of Reclamation (Grant No. 4-FG-60-04370) 
and the Texas Water Development Board (Research & Planning Fund Contract No. 94-483-046) to The 
University of Texas at Austin, Marine Science Institute. 

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Appendix E ♦ E-9 



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199-216 



E-10 ♦♦♦ Utili^tion of 'Estuarine Organic Matter DuringGrowth 
and Migration by juvenile Brown Shrimp 



Table 1 : Average stable isotope values for food sources from different habitats 



Sources 



6"C 



6'=N 



Nueces River 


Riverine POM 


-27.6 ± 0.9 (n=6) 


9.5±1.0(n=3) 




Riverine SOM 


-23.2 ± 0.9 (n=3) 


4.7 (n=1) 




Fraxinus sp. 


-28.5 ± 0.6 (n=5) 


6.6 (n=1) 




Salix sp. 


-29.8 ± 0.4 (n=6) 


8.3 ±1.1 (n=3) 




Phragmites sp. 


-15.2 ± 0.6 (n=5) 


-1.6 (n=1) 


Rincon Bayou 
Marsh 


POM upper marsh 


-21.2±2.8(n=18) 


5.5±2.2(n=10) 




POM lower marsh 


-22.8 ± 0.8 (n=2) 


6.5 ± 0.9 (n=2) 




Marsh SOM 


-21.2 ± 0.5 (n=7) 


7.6 ± 0.7 (n=2) 




Marsh Grass 


-15.6 ±1.1 (n=3) 


7.6 (n=1) 




Benthic diatoms 


-17.5 ± 0.9 (n=5) 


4.6 ± 1 .9 (n=2) 


Rincon Bayou 
Mouth 


POM marsh 


-24.3 ± 2.9 (n=4) 


9.3 ±0.1 (n=2) 




Spartina sp. (fresh leaves) 


-16.0 ± 0.3 (n=2) 






Spartina sp. (old leaves, 
stems) 


-14.5 ± 0.6 (n=6) 


2.6 (n=1) 




Salicomia sp. (fresh leaves) 


-27.3 ± 0.4 (n=4) 






Salicomia sp. (old leaves, 
stems) 


-27.1 ± 0.6 (n=5) 


5.4 ± 1 .9 (n=3) 




Zooplankton 


-26.0 ± 0.3 (n=3) 


11.7(n=1) 


Aransas Pass Inlet 


Ocean POM 


-23.5 ± 0.9 (n=8) 


8.1±3.0(n=5) 



Appendix E ♦ E-11 



Table 2: 


Spatial and temporal changes in stable 


isotope compositions 


of Brown Shrimp 








Locations 






Dates 


Aransas Pass Rincon Bayou 
Inlet Mouth 


Rincon Bayou 
Lower Marsh 


Rincon Bayou 
Upper Marsh 


Nueces River 


(1996) 


5"C 5''N 6"C S'^N 


5 "C 6 '^N 


(5 "C 6 "N 


6 "C 6 ''N 



Jan 25 


-20.7 


7.7 


Feb 9 


-20.7 


5.5 


Mar 4 


-21.7 


5.8 


Apr 10 






Apr 25 






May 14 






May 30 







-19 3 to 3.4 

-18.0 

-19.3 to 4.8 to 8.7 -17.9 to 4.5 

-14.8 -14.5 

-21.3 to 1.0 to 6.6 -18.3 to 1.2 to 5.1 -25.2 to 6.6 to 9.9 
-17.0 -15.3 -23.9 

-20.3 to 6.5 to 10.2 -20.4 to 5.2 to 9.9 
-16.3 -15.5 

Jun2 -18.0 to 1.3 to 2.3 -23.2 to 9 5 to 11.0 

-18.4 -21.7 

Jun 19 -21.8 to 10.1 to 

-20.4 13.9 

Jun 21 -20.6 to 4.5 to 13.1 -20,1 to 5.6 to 6.4 

-18.4 -16.5 

Jul 29 -16.4 to 3.3 to 9.7 
-12.5 



E-12 ^ Utili^tion of Rstuarine Organic Matter DuringGnwih 
and Migration by Juvenik Broivn Shrimp 




Figure 1 : Locations of sampling sites in various shrimp habitats. 



Appendix E ♦ E-13 



100 
90 
80 
70 

g 60 

B 

5 50 
so 

J 40 
30 H 
20 

10 



Penaeus aztecus 



A 






■ 

•A 
D 



« Aransas Pass | 

Rincon Bayou 

■ u 

mouin 

i Down Marsh 

X Up Marsh 

• Nueces River 



T 1 1 1 1 1 1 1 1 1 1 1 1 1 — 

16/01 31/01 15/02 01/03 16/03 31/03 15/04 30/04 15/05 30/05 14/06 29/06 14/07 29/07 

Sampling dates 
Figure 2: Size of brown siirimp Penaeus aztecus caught on sampling dates, in different locations. 



E-14 ^* Utilisation ofEstuarine Organic Matter During Growth 
and Miff-ation hy Juvenile Brown Shrimp 





T Spartina 
Panicum virgatum 


spartinae 


Blue 


green 

f 


algae 


( • (living) 
j§ (detrital) 






-15 - 
-16 - 


Sources 








-17 - 
-18 - 








(» 






Spartina altemiflora 




-19 - 






Benthic diatoms 








-20 - 


















-21 - 


















U-22- 
CO -23 - 
















^j POM 


-24 - 
















ocean 


-25 - 


















-26 - 














Salicornia sp 




-27 - 
-28 - 
-29 - 


( ( Fraxinus sp 










(living) (detrital) 




-30 - 


^ 


Salix sp 














-31 J 








1 — 






1 


1 ' 







N 


lueces River 


Rincon Bayou 


Marsh 


Rincon Bayou Aransas Pass 




-n 






Mouth 




B - 






Sub-adults (80-90 mm) 




-12- 










(July 29) 














-13- 




Penaeus aztecus 




■ 


■ 










_ ocean 




-14 - 






II --^ 1 








(April 10-June 21) 


4 






-15- 






/ 














r ' ^ 


. 




-16- 




-I- 






u 


-17- 


X 


II 


r 


-| 


r -| 


r 


-| 


11 ' 

P / 

/ 


CO 


-18- 


• 


-f/^-*^ 


1 


1 










/ 




-19- 


^ / 


■ ~- 1 


1 


1 


'l 


' 1 1 V- 


:1 1 


II 




-20- 


/ 


-^ J 


. 


. 






-21 - 


'I '' 




^ ~^ ■ * - 






' 1 

i 


1 / 




-^ ocean 




-22- 


1 , 


f. 1 
1 








-23- 


\ 1 


\ }(■ 




(January 25-March 4) 






X 




Larvae (10- 1 1mm) 




-24- 


II 








-25- 










(May 14-June 19) 








-26- 






, , 



Nueces River Up Marsh Down Marsh 



Rincon Bayou Aransas Pass 
Mouth 



Figure 3: Variation of carbon Isotopic ratios in different feeding habitats. A. Potential food sources. 8. 
Juvenile brown shrimp Penaeus aztecus as they migrate through ecosystem as indicated by dotted lines with 
arrows. 



Appendix E <* E-15 



5i^N (%o) 
10 



6 - 



4- 



Aransas Pass 



515N (%o) 
14 



— I 1 1 1 1 1 1 1 1 — 

10 20 30 40 50 60 70 80 90 100 




10 20 30 40 50 60 70 80 90 100 




14- 










-♦■ 




• 


Nueces 


River 






121 














• 








♦ ♦ 






10- 








♦\ 




♦ 










♦ 






8- 








♦ 














♦ 






% 


) 10 


20 


30 


40 50 


60 


70 80 90 IC 



Length (mm) Length (mm) 

Figure 4: Relationship between 6"N and length (mm) of brown shrimp Penaeus aztecus in different 
habitats. 



E-1 6 V UtiJi^ation o/Esluarine Organic Matter During Growth 
and Migration hy Juvenile Broum Shrimp 



APPENDIX F 



Effects of Temporality, Disturbance 
Frequency and Water Flow on an 
Upper Estuarine Macroinfauna 
Community 



Christine Ritter Texas Water Development Board, Austin 

Paul A. Montagna University of Texas at Austin, Marine Science Institute (corresponding author) 

Submitted for publication to Marine Ecology Progress Series (draft date: January 14, 2000) 

ABSTRACT: The effects of disturbance frequency and altered flow on estuarine macrobenthic community 
structure and colonization were studied ia Rincon Bayou, a low-inflow, microtidal, shallow habitat in the upper 
Nueces Delta, Texas. Three flow treatments were used: increased flow (within a weir), natural flow (control), 
and decreased flow (between nets). Four disturbance frequency treatments were used: undisturbed, biweekly, 
monthly, and bimonthly disturbance. Disturbance was imitated by placement and replacement of 6.5-cm 
diameter trays filled with defaunated sediment. Significant flow velocity differences were found among flow 
treatments, but turbidity, abimdance, biomass, and diversity were not different. Significant differences among 
disturbance frequencies were found. Abundance and biomass decreased with increasing disturbance frequency 
indicating post-disturbance community persistence is important in regulating commimity structure. In the flow 
experiment, structvire and tray effects were evident because abundance and biomass were higher near structures 
and in trays than in controls. The higher abundance and biomass in defaunated sediments relative to 
background sediments indicates disturbance plays an important role in community production of early 
succession commimities. Collection date was the most important determinant of community structure, thus 
natural variability overwhelmed effects of both experimental manipulations. The temporal changes were driven 
by a Streblospio benedicti recruitment event (resulting in densities as high as 1.3 10*^ m^ captured 20 June 1997, 
and a flood event that began 22 June 1997. After the flood, S. benedicti density declined rapidly and freshwater 
species invaded, leading to three distinct community states over the 14-week period of the study. The 
overwhelming significance of "temporality" (i.e., short-term temporal change in commvinity structure) is the 
unexplained temporal component of community variation in experimental manipulations. Temporality is 
simply a smaller temporal scale than seasonality. 

KEY WORDS: benthos, disturbance, estuary, flow, macrofauna, Streblospio benedicti 

INTRODUCTION 

Flow dynamics in estuaries result from complex interactions among tidal cycles, estuarine circulation, 
hydrology, cUmatological forcing, and rate of freshwater inflow. Flow dynamics may vary over the coiu:se of a 
day, month, season, or on climatological scales. Characteristic flow patterns of an estuary depend on its size, 
shape, and location. Numerous mechanisms exist by which an estuary's flow regime may affect macrobenthic 
populations. Changes in flow velocity can affect the sediment flux of ammonium (Asmus et al. 1998), depth of 
oxygen penetration (Beminger and Huettel 1997), and benthic aerobic respiration and chemical oxygen demand 
(Boynton et al. 1981). Flow velocit)' also plays an important role in larval dispersal and settlement (Butman 
1986a, 1986b, 1987, 1989), food availability (Smaal and Haas 1997), and suspension feeder growth rates 
(Eckman and Duggins 1993, Hentschell 1999a, 1999b). The pattern of flow dynamics (or frequency of change) 



Appendix F ♦ F-1 



chaiactensdc of an estuary may play an important role in determining macrobenthic community structure 
(Pearson and Rosenberg 1987). At^'pical changes in flow dynamics, such as those occurring during storms, may 
also be important determinants of benthic commumt}' structure (Renaud et al. 1996). 

Physical disturbances (e.g., dredging, trawling, or major storms) affect macrobenthic communities within the 
context of the flow regime and ambient environmental conditions. ITie frequency of disturbance may be a 
major determinant of community structure. Connell (1 978) h)'pothesi2ed that more frequent disturbances will 
maintain a commumty at an earlier stage of succession, which is characterized by lower diversit)', abundance, 
and biomass than reference communities of lower disturbance frequencies. Because flow dynamics can vary on 
short temporal scales, it could be an important physical regulator of community change. 

The purpose of this study was to determine temporal effects of disturbance frequency and altered water flow 
on macroinfauna early succession. Macrobenthic recruitment and community structure were expected to 
change for each flow treatment (Rhoads and Young 1970). Increased flow was expected to promote 
suspension feeder populations and growth rates (Hentschel 1999a; 1999b). Communities subjected to greater 
frequencies of disturbance were expected to have lower diversity, abundance and biomass than those subjected 
to less frequent disturbance as predicted by Connell (1978). Early succession was expected to be dynamic and 
vary among flow treatments because the dominant organism at the study site, Streblosio benedicti . is an 
opportunistic species (Grassle and Grassle 1974), can recruit in excess of 25,000 m ' a month (Levin 1984), and 
can increase growth rates with flow speeds between 9 and 18 cm s' (Hentschel 1999b). 



METHODS 

Site Description 

Rincon Bayou is a tidal creek in the Nueces-Corpus Chnsti Bay estuarme ecosystem. Rincon Bayou lies west of 
Nueces Bay and north of the Nueces River. It may have once been the primary channel of the Nueces River 
but is now connected to the Nueces River via an overflow channel constructed in October 1 995 (Irlbeck and 
Ergerl998). 

Rincon Bayou is a low-inflow environment, subject to hypersalinity during droughts and freshets during floods. 
Monthly discharges from the Nueces River into Rincon Bayou ranged from -1200 to 300,000 m' between May 
and December, 1996, but about 60 % of daily discharges were < ± 1200 m' (USGS unpublished data 1997). 
Rincon Bayou receives little tidal influence (< 0.2 m) and water movement is predominandy wind driven (Salas 
1994), or inflow driven during floods. 

The location within Rincon Bayou, station C (27° 53.927' N, 97° 36.250' W), was chosen because it lies 
approximately along the axis of prevailing winds, which blow from the southeast and south (Port of Corpus 
Christi Authority 1993). In addition, station C is part of a quarterly sampling program begun in 1994 from 
which a historical context can be drawn. Surface sediment is approximately 86 % silt and clay, and contains 0.7 
% total organic carbon. Sahnity averages 40 %o, but has ranged between 0.2 - 160 %o over a four-year 
monitoring period (Montagna unpublished data). Dissolved oxygen averaged 7.36 mg 1 ', but has ranged 
between 1.01 - 15.32 mg 1 ' over the same period. Six macrobenthic taxa were commonly found at station C: 
the polychaetes Streblospio benedicti Webster and Laeoneris culven (Webster), the crustaceans Ostrocoda and 
Hemicyclops sp., and insect larvae of Ceratopogonidae and Chironomidae. 

Experimental Design 

The goal of the present study was to determine the effects of flow alteration and disturbance frequency on 
short-term changes of macrofauna (Fig. 1). Three flow treatment levels were used: increased flow, decreased 
flow, and control. Increased flow was effected through the use of weirs parallel to currents. Weirs are used to 
increase water pressure, and hence flow, over a specific area (Fig. 2a). Decreased flow was effected through the 
use of nets perpendicular to currents. Nets are commonly used to reduce wave energ)' by slowing water 
velocity. Control flow was areas perpendicular, but away from structures and represents the natural flow 
conditions of the Bayou (Fig. 2b). 



F-2 V Effects ofTemporality, Disturbance Frequency and Water Flow 
on an Upper Estuarine Macroinfauna Community 



Two plots of each flow treatment were constructed (Fig. 1). The plots, thus represent replication at the 
treatment level. Two control plots, 1 m x 1 m area, were delineated by PVC comer poles. Two weir plots 
(increased flow) were constructed of 1 m x 0.5 m, 3/4" (1.9 cm) plywood attached to 1.5 m PVC poles (Figs. 1 
and 2). Net plots (decreased flow) were 2 m long and 1 m wide and made of 2.5 cm mesh attached to 1 .5 m 
PVC poles (Fig. 2a). All PVC poles extended 1 m into the sediment. The open faces of the weirs and the long 
sides of the net faced southeast, the direction of predominant tidal and wind driven flow (Montagna et al. 
1998). All experimental sampling areas were 1 m x 1 m squares within the central region of each structure 
(Fig. 2a). 

Structures were biult within a 0.25 km^ region (Fig. 2b). Furthest downstream were a weir plot and a net plot. 
Fxirthest upstream were a control plot and a net plot. In between were a weir plot and a control plot. Plots 
were staggered to Umit possible structure interactions and were all roughly the same water depth. Structure 
interactions are unlikely because Rincon Bayou is generally a low-inflow, microtidal environment. 

Frequency of disturbance was varied. Four distiubance frequency treatment levels (i.e., undisturbed, biweekly, 
monthly, and bimonthly) were nested within each flow treatment plot (Fig. 1). Disturbance was imitated by 
emplacement of defaunated sediment in trays. Undisturbed sediment was the control for the disturbance 
frequency treatment. Bi-weekly disturbance was the most frequent disturbance investigated and trays remained 
in the field for 2 weeks. Monthly disturbance was the moderate frequency level and trays remained in the field 
for 4 weeks. Bimonthly disturbance was the least frequent disturbance investigated and trays remained in the 
field for 8 weeks. Short-term frequencies of disturbance were used to determine effects on development of 
early succession communities. 

Disturbance ttays were constructed from 6.5-cm diameter acrylic tubes, 3 cm high, and fused to 7 cm x 7 cm 
square bottoms. Trays were filled with sediment collected from station C and defaunated using a microwave. 
Approximately 500 ml of sediment were microwaved for 1 5 minutes to bring sediment temperatures to roughly 
100 °C. Microscopic investigation of subsampled sediment yielded no live animals following defaunation. 
Trays were filled with well-mixed defaunated sediment in the field, and placed at predetermined random 
positions within the experimental area of each flow plot. Flagged rods were used for treatment identification 
and to avoid tray loss. Only one tray was turned over and lost during the study. 

The disturbance frequency experiment was replicated two different ways: by initial placement date (23 April and 
04 June), and by ending sampling date (20 June and 01 August) (Fig. 3). This required eight sampling dates over 
a 14- week period (Fig. 3). On each sampling date, three replicate tray samples and three undisturbed samples 
(the top 3 cm of a 6.5-cm diameter core) were taken in each disturbance frequency level (Fig. 1). 

Measurements 

Hydrographic data (e.g., salinity, temperature, dissolved oxygen) were collected from each flow treatment plot 
on each field date using a Hydrolab 4000 Sonde. Three turbidity samples were collected from each flow 
treatment plot and analyzed using a HACH model 2100A turbidimeter. Rainfall, tidal stage, and flow volumes 
were collected from a gage placed at the mouth of Rincon Bayou where it connects to the Nueces River 
(unpubUshed data collected by the United States Geological Survey at Station 08211503, Rincon Bayou Channel 
near Calallen, Texas). 

Flow velocity was measured twice for each flow treatment plot and level to determine the effect of the 
structures. How velocity was measured only twice because flooding prevented equipment deployment and 
retrieval on most dates. Flow was measured on 07 May and 17 July 1997 using a UNIDATA Starflow 
Ultrasonic Doppler flow meter. Velocity, depth, and temperature were recorded every two minutes for 6 to 
24 minutes per flow plot per date while sediment samples were being taken from the adjacent flow plot. 

Macro fauna were extracted from sediment using a 0.5-mm sieve, identified to the lowest taxonomic level 
practicable, and counted. Lecithotrophic larvae were assumed to be Streblospio benedicti . which are more 
common at Rincon Bayou than the alternative Polydora comuta (Montagna and Ritter unpublished data). 
Biomass was determined for each taxonomic group by drying samples for 24 hours in a 55 °C oven and 
weighing to the nearest 0.01 mg. Abundance and biomass were converted to a per meter basis and (natural 
logarithm) transformed for statistical analysis as follows: In (n m"^-l-l) and In (g m'^+1) respectively. Hill's 
diversity (Nl), and Peliou's evenness (El) were calculated to summarize community structure characteristics. 

Appendix F ♦ F-3 



Diversity was calcvilated as Nl = (exp) , where IT = 2 (p, In p), where p, = n, / n, where a, = abundance of 
species i, and n = total abundance (Ludwig and Reynolds 1988). Evenness was calculated as El = ln(Nl) / 
ln(NO), where NO = total number of species (Ludwig and Reynolds 1988). Nl is the number of abundant 
species, and El is the familiar J . 

Statistical Analysis 

A two-way analysis of variance (AN OVA) was used to determine turbidity and velocity differences among flow 
treatments and sampling dates. More complex ANOVA was used to test for differences in macrobenthic 
response among flow, disturbance frequency, and sampling date treatments. All ANOVA models were 
calculated using SAS GLM procedures (SAS 1985). 

A three-way, incomplete factorial, randomized block design was tised to test for community differences in over 
all sampling dates (Fig. 1). Main effects (i.e., treatments) were flow, disturbance frequency, and sample 
collection date. The randomized block was flow plot. Blocks are replicates that do not have interactions with 
main effects. The design is incomplete factorial because distiorbance frequency treatments were not started or 
ended on the same dates. Because some date cells are missing in the biweekly, monthly, and bimonthly 
frequency levels, the frequency*date interaction and flow* frequency* date interaction do not exist in the 
ANOVA model (Table 1). In this model, date is more like a block (controlling nuisance variation) than a main 
effect. 

Full rank, three-way ANOVA models do exist for subsets of the data set (Fig. 3). The experiment was 
replicated two different times: as starting dates and as ending dates. So, macrobenthic data was analyzed with a 
three-way randomized block design twice: Once for trays deployed on two dates: 23 April and 4 June 1997, and 
once for trays collected on two dates: 20 June and 1 August 1997. These analyses are referred to as "initiating" 
and "ending" sampling dates respectively (Table 1). 

Interpretation of results from complex ANOVA designs is often obscured by significant interactions, because 
the main effects tests are invalid. To simply interpretation of the present study, two-way, incomplete, 
randomized block models were calculated by a treatment level. Examination of the simple main effects allows 
testing and interpretation of the first main effect at all levels of the second main effect. The simple main effects 
models were calculated by disturbance frequency for flow and sampling date treatments with plot as a 
randomized block. Tukey multiple comparison tests were used to determine differences among levels of flow 
or distiirbance frequency cell means. The implementation of Tukey uses the harmonic mean of cell sample 
sizes when sample sizes are unequal (SAS 1985). \'ariance components analysis was used to estimate the 
percent of variation attributable to each main and interaction effect in all ANO\''A models. 

Principal component analysis (PCA) was used (SAS 1985) to determine treatment effects on species 
composition. The covariance matrix of log transformed species abundance, standardized to a normal 
distribution was used for PCA. Using the covariance, ittstead of the correlation matrix, eliminates problems 
encountered where many rare species with zero counts exist. The multivariate PCA method is a species 
dependent analysis of community structure, unlike the species independent analysis of diversity indices. 

RESULTS 

Hydrography 

Hydrographic conditions at station C varied during the course of the experiment (Fig. 4). The small (about 
5 %o) drop in salinity from 23 April - 22 May 1997 is due to local rainfall. Average salinity peaked on 20 June 
1997 at 25.6 %o, but dropped to %o on 2 July. The fresh conditions correspond with a flood event that began 
on 22 June. The flood resulted primarily from rain in the watershed northwest of the delta. Dissolved oxj'gen 
was highest 23 April and 7 May, but lowest 4 June. On 4 June, dissolved oxygen data collected prior to 
9:30 a.m. were 2.4 mg 1"' at weir 1, and 2.88 mg 1 ' at net 1, indicating hypoxic conditions probably occurred 
during the previous night. Temperature generally increased with onset of summer. 

A significant (p = 0.0020) flow*date interaction was present because there were greater differences among flow 
rates on 17 July than on 07 May (Fig. 5). On both dates, weir structures had increased flow velodries, and the 

F-4 ^ Effects of Temporality. Disturbance Frequency and Water Flow 
on an Upper Estuarine Macroinjauna Community 



net structiues had decreased flow velocities relative to control plots. Average flow velocities were highest in 
between weir structures (132 mm s '), were lower in control plots (102 mm s '), and lowest in between net 
structures (75 mm s '). There was no difference in flow treatments between the two replicate plots. 

Turbidity did not vary significandy among flow treatments, indicating flow manipulations did not alter the 
concentration of suspended sediment. Turbidity differences were found among dates (p = 0.0001). No 
interaction effects were detected. Turbidity samples for 23 April were not included in ANOVA because 
sediment was resuspended during sample collection. 

Treatment Effects 

Macroinfauna community structure response over all sampling dates was clear, because there were no 
significant treatment interactions for diversity (Nl), and evenness (El). There was no significant differences 
among flow treatments for diversity, and evenness. There were significant differences for disturbance 
frequency levels for diversity (p = 0.0001), and evenness (p = 0.005). Significant differences (0.0001) among 
dates were detected for diversity and evenness. Diversity was highest for biweekly samples (1.82) and lowest 
for undisturbed samples (1.31); both of which were significandy different from monthly (159) and bimonthly 
(1.62) samples, which were the same fTable 1, Tukey test). Biweekly (0.59) samples had the highest average 
evenness, which was different from monthly (0.44), bimonthly (0.37), and undisturbed levels (0.28) (Table 1, 
Tukey test). Evenness in monthly and bimonthly samples were the same, and bimonthly and undisturbed 
sample means were the same (Tukey test). 

Macroinfauna standing stock (i.e., abundance and biomass) response to the experimental treatments was 
analyzed three ways: by all sample dates, by initiation dates, and by ending dates (Table 2). Regardless of the 
design or analysis used, there were many significant interactions obscuring interpretation of main treatment 
effects ^able 1). Total biomass and abundance did not have si milar results in regard to which interactions 
existed. The two main experimental treatments (flow and disturbance frequency) contributed very litde 
variance components regardless of the analysis technique, ranging from % to 12.7 %. 

In general, the full-rank analysis for two ending dates yielded similar results to the incomplete factorial design 
for aU sampling dates for both abundance and biomass (Table 1). The difference between the contribution of 
the date effect is particularly striking. Sampling date contributed the largest percentage of variance for 
abundance and biomass for the incomplete factorial analysis and the fiill-rank analysis of ending dates, ranging 
from 35.4 % - 86.9 %. In contrast, date contributed no variance to the full-rank analysis of initiating dates, and 
nearly all the variance was contributed by the frequency*date interaction for abundance (91.9 %) and biomass 
(57.7 %). The similarity between analyses of p-values and variance components for aU dates and ending dates 
indicates macrobenthic response is similar among all treatments on given dates. S imil arly, sigmficant 
frequency*date interactions found in the initial date analysis indicates that commumty change is different for 
each experimental treatment due to samples being taken on different dates. 

Regardless of the analysis method, the flow*frequency interaction was always significant (Table 2). The nature 
of the interaction is different responses to flow variation in frequent disturbances (at biweekly and monthly 
scales) compared to less frequent disturbances at longer times scales ( bimonthly and undisturbed) (Fig. 6). 
Abundance was very similar in flow treatments during short-term disturbance, but the increased flow treatment 
had much higher abundances in the samples that had recovered the longest or were undisturbed (Fig. 6a). The 
trend for biomass was similar, except that biomass was higher in the decreased flow treatment than in the 
control or increased flow treatment over the short-term disturbances (Fig. 6b). 

Simple Main Effects 

Analysis of the full rank model yielded many significant interaction effects (Table 2), so simple main effects 
models were run (Tables 3 and 4). Flow treatment effects were determined with separate analyses by each 
disturbance frequency level (Table 3) and frequency treatment effects were determined with separate analyses 
by each flow level (Table 4). 

Flow Effects 



Appendix F ♦ F-5 



The flow tieatment tests are generally valid because of non-significant interactions with date, except for the 
undisturbed samples (Table 3). No sigmficant abundance differences were found among flow treatments. The 
biomass differences among flow treatments for biweekly and monthly levels (Fig. 6.) were significant (Table 3). 
At the biweekly and monthly frequency levels, decreased flow yielded significandy greater biomass than control 
and increased flow levels (Tukey test). Abundance changed over collection dates for all frequency levels. 
Biomass changed over the long-term, but over short-term (biweekly) time intervals. 

The flow*date interaction was significant for abundance and biomass for the undisturbed treatment (Table 3). 
This interaction is especially interesting because it represents how undisturbed sediment changed over time as a 
result of the flow treatments alone (Fig. 7). In undisturbed sediment, highest abundance and biomass values 
were found within structures that altered flow. Except for the first collection date, the abundance and biomass 
in the control plots were always lower than the altered flow plots indicating a possible structure effect. So, the 
interaction was due to the first collection date (when all flow levels were similar) and the similarity' of flow 
alteration structures (which alternated between highest and second highest values). 

Frequency Effects 

Disturbance frequency tests were affected by significant frequency*date interaction effects, except in decreased 
flow structures ^able 4). At the decreased flow level, average biomass of monthly (1.20 g m "), bimonthly 
(1.15 g m^, and weekly (0.90 g m^ frequencies were similar to each other, but were significandy different from 
undisturbed average biomass (0.69 g m ■^. 

Effects of the disturbance frequency experiment on the natural communit)' is represented by changes in the 
undisturbed sediment communit)' in control level of flow treatments. There was a significant frequenc)'*date 
interaction for control flow levels for both abundance and biomass (Table 4). Average abundance and biomass 
was lower in undisturbed sediment than in all disturbance treatment levels except for biweekly samples 
collected on 07 May (Fig. 8). The lower abundance and biomass in undisturbed samples may indicate 
disturbance enhances community productivity'. On most dates, average abundance and biomass in bimonthly- 
frequency, control-flow, samples were greater than other disturbance frequency samples. In general, there was 
a trend of increasing abundance and biomass from biweekly, to monthly, to bimonthly distixrbance frequencies, 
indicating growth or recruitment with time. 

Community Structure 

Ten taxa were found during the experiment. Streblospio benedicti and Laeoneris culveri were present on each 
sampling date. Chironomid larvae were observed 23 April and 7 May, but appeared in abundance 20 June, 
persisting through 1 August. Polydora comuta Bosc was present between 4 June and 2 July, but was most 
abundant on 20 June. Polydora comuta was observed only once in 31 observations at the control- flow plot 
indicating a possible positive structure effect. Mysidopsis aknyra Bowman was present between 23 April and 
22 May, and was found only in undisturbed samples. Nermerteans were found on 20 June and 02 July in 
undisturbed and monthly samples. Mulinia laterahs (Say) was foimd infrequendy between 23 April and 20 June 
in undisturbed sediment of decreased and control flow treatments. Mediomastus ambiseta (Hartman) was 
fotmd infrequendy between 4 June and 1 August. Amphipods and oligochaetes were each present in a single 
sample during the course of the study. 

Average abundance and biomass were greatest for samples collected on 20 June 1997 (Figs. 7 and 8). Average 
abundance was 422,000 m'^ and average biomass was 5.62 g m'^. Streblospio benedicti accounted for 96 % of 
abundance and 76 % of biomass in these samples on average. The bimonthly- frequenc}', increased-flow plots 
contained the greatest average abundance and biomass of all samples in the study. For example, one sample 
contained 4,730 organisms (1,342,000 m ■^, of which 4,667 (1,324,000 m "') were S. benedicti . Many S. benedicti 
were very small, presumably lecithotrophic larvae or post larval juveniles, but sizes were not measured. 

Principal component analysis (PCA) yielded three distinct collection date groups (Fig. 9a), but no flow or 
disturbance frequency effects. PCI separated samples collected 20 June and 02 July from those collected 
17 July and 01 August. PC2 separated samples collected 23 April - 04 June from those collected 20 June - 
01 August. Chironomid larvae (CL) loaded 0.97 on PC2 (Fig. 9b) indicating their importance to characterizing 
communities samipled 20 June and later. Streblospio benedicti and Laeoneris culveri loaded positively for both 



F-6 V Effects of Temporality, Disturbance Fnquengi and WaterFbw 
on an Upper Estuarine Macroinfauna Community 



PCI and PC2 indicating their importance to characterizing samples collected 20 June and 02 July. PC A of each 
date group separated in the first PCA also failed to segregate flow treatments or disturbance frequency 
treatments. 

DISCUSSION 

Flow Effects 

The structures used to manipulate flow significandy altered flow velocity in the expected manner (Fig. 5), but 
they did not significandy affect tvirbidity or macrobenthos overall (Table 2, Fig. 6). Flow treatments accounted 
for only a small fraction of the total variance in abundance and biomass (<1.9 %). Significant interactions with 
sampling date and disturbance frequency treatments indicate that altered flow affected macrobenthos only in 
certain instances (Table 2). The strongest flow effects were found in undisturbed samples only (Table 3). In 
undisturbed sediments, average abundance and biomass were greater at increased and decreased flow levels 
than in control plots for all sampling dates except the first (Fig. 7). Increased abundance and biomass in 
increased and decreased flow plots relative to control plots indicate the physical presence of experimental 
structures may have affected macrobenthos. It is possible structure proximity afforded the community a refuge 
from predation or increased food supply by increasing sedimentation rate. The difference may also explain the 
flow*frequency interaction, because fi-equentiy disturbed (biweekly and monthly) samples had low abundance 
and biomass due to recent defaunation. 

At the species level, Polydora comuta was found only at increased and decreased flow plots with one exception, 
indicating a possible attraction to structures. Between 1994 and 1997, P. comuta was not observed at Rincon 
Bayou (Montagna and Ritter unpublished data). The present study is the only report of its presence at 
station C. 

Failure to detect significant abundance and biomass differences among flow treatments may arise from 
inadequacy of the manipulations. Flow treatments did not alter flow velocity enough to significandy affect 
resuspended sediment. Average flow velocities of weir treatment plots ranged between 10.1 and 16.4 cm s'. 
These velocities may not have reached critical erosion velocities of the natural cohesive sediment of Rincon 
Bayou. For example, critical erosion velocities of the Skeffling intertidal mud flat on the Hiunber Estuary, U.K. 
ranged between 21.8 cm s ' and 30.8 cm s ' (Widdows et al. 1998). Community change driven by altered 
resuspension rates would not be detected by the manipulations in the present study. 

Changes in flow velocity affects growth of some species. For example, growth of some macrobenthic 
suspension feeders, such as Membranipora membranacea . Balanus crenatus . and Pseudochitinopoma 
occidentalis . are inhibited by flow velocities 12-30 cm s ' (Wildish and Kristmanson 1979; Eckman and 
Duggtns 1993). In contrast, growth rates of spionid polychaetes common to Rincon Bayou increase with 
increasing flow. Polydora comuta and Streblospio benedicti increase growth rates with flow velocities between 
9 and 18 cm s ' (Hentschel 1999a; 1999b). For example, P. comuta grew at a rate of 1.4 volumetric doublings 
per day, reaching sexual maturity within 1 week at 18 cm s ' velocity (Hentschel 1999a; 1999b). Because the 
response by the two spionid polychaetes common to the study area increased over the range of measured flow 
velocities, inhibition probably did not occur near structures. It is more likely slight enhancement occurred, 
explaining the small increase of abundance and biomass in increased flow plots (Fig. 7). 

The increase of abundance and biomass in increased flow (weir structure) plots is due to changes in population 
size of Streblospio benedicti . Population growth of S. benedicti appears to have been enhanced at increased 
flow plots prior to the flood of 22 June 1997. On 20 June, S. benedicti populations at increased flow plots were 
twice that of decreased flow and control plots. Streblospio benedicti grows faster under higher flow conditions 
(Hentschel 1999a; 1999b). The unique life history of S. benedicti could also be partly responsible for the rapid 
population growth. Colonists could have been adults carrying a full brood (Levin 1984). 

Disturbance Frequency Effects 

Disturbance frequency treatments appear to demonstrate succession. Longer periods between disturbances 
(i.e., biweekly to monthly to bimonthly) were associated with increasing abundance and biomass and decreasing 
evenness (Table 1, Fig. 8). The trend demonstrates early colonization and successively greater abundance, 
biomass, and less dominance with longer periods between disturbance. Lower evenness and abundance in 



Appendix F ♦ F-7 



more frequently disturbed communities indicates greater disturbance frequency maintains the community at a 
earlier stage of succession (Pearson and Rosenberg 1976; 1978; Dauer 1993; Trueblood et al. 1994; Weisberg et 
al. 1997). If a commumt)' is disturbed every two weeks, it cannot proceed beyond the coloiuzation stage. Thus, 
the length of time since the last disturbance regulates the communit)' (Connell 1978). In addition, communities 
subjected to altered flow disturbance (i.e., increased or decreased) had higher abundance and biomass than 
natural (control) communities (Fig. 7) explaining the frequency*flow interaction (Fig. 6). These results indicate 
disturbance may play an important role in regulating macrobenthic community dynamics and increase 
secondary' production as suggested by Rhoads et al. (1978). 

.\11 disturbance frequenc)' levels, including undisturbed ambient samples, were dominated by the polychaete 
Streblospio benedicti . Streblospio benedicti is an opportunistic species (Grassle and Grassle 1974). 
Dominance by opportunists is a key characteristic mdicating early succession, or highly disturbed, estuanne 
macrobenthic communities (Pearson and Rosenberg 1976; 1978; Thistle 1981; Dauer 1993; McCook 1994; 
Weisberg et al. 1997). Dominance of S. benedicti in the current study may indicate the community of station C 
is highly disturbed by natural environmental variation, e.g., as the broad salinity ranges and flow conditions 
foimd during this study (Fig. 4). 

Community abundance, biomass, and diversity of undisturbed sediments were lower than that of all disturbance 
treatments after the first (7 May) biweekly samples (Fig. ). The undisturbed sediment communit)' is the 
ambient, reference community against which coloni2ation of defaunated (disturbed) sediment was compared. 
Increasing abundance, biomass, and diversity were expected in defaunated sediment until disturbed and 
undisturbed communities were similar. In contrast, abundance of the bimonthly disttirbance frequency level 
was 8 times that of the undisturbed community; biomass of bimonthly level was twice that of undisturbed 
sediment; and diversity of bimonthly level was higher than that of undisturbed sediment. 

There are three possible explanations for the differences between undisturbed and defaunated communities. 
The structure of experimental trays may attract organisms as a refuge or alter water flow affecting deposition 
and recruitment (Butman 1986b; Snelgrove et al. 1993). The defaunated sediment may attract macrobenthos or 
promote macrobenthic reproduction. Macrobenthic succession of defaunated sediment may be mitiated by a 
poptJation burst of opportunistic species. The population burst may exceed late succession commumt)' 
abundance and biomass, but return to normal levels after a period longer than 8 weeks. One, two, or all three 
explanations may be responsible for the observed differences. 

Temporality 

Temporality, "the quality or state of being temporal (Mish 1985, p. 121 4)", is a property of communities that 
arises in all studies from the complexity of ecological interactions in the natural environment. Communit)' 
variation through time is not in itself a new finding. Temporalit)', however, is the unexplamed temporal 
component of communit)' variation that may exceed that of experimental treatments. Examples of temporality 
can be found in benthic communities of the Savin Hill mudflat, Boston Harbor (Trueblood et al. 1994), 
microbial communities of the Parker River salt marsh, Rowley, MA (Montagna and Ruber 1980), and epifaunal 
communities at Beaufort, NC (Sutherland and Karlson 1977; Holm et al. 1997). In all these cases, natural 
commumt)' variation over rime (i.e., temporality) exceeded the effects of experimental mampulations. 

In the present study, collection date had the strongest effect on macrobenthic abundance (Figs. 7 and 8; 
Tables 2 - 4), biomass (Figs. 7 and 8; Tables 1 - 4), diversity, and community structure (Fig. 9). Thus, natural 
temporal variation of salinity, temperature, and dissolved oxygen (Fig. 4), which may be associated with 
seasonality and flooding, played a greater role in determining community structure than flow or disturbance 
frequency treatments (Tables 2-4). There were three community states through time (Fig. 9). The first group, 
representing 23 April - 4 June 1 997, was dominant when average salinity varied between 1 1 and 1 8 %o, average 
dissolved oxygen declined from 11.22 mg 1 ' to 3.89 mg 1 ', and temperature varied between 24.5 and 26.5 °C 
(Fig. 4). The low abundance and biomass estimates of 4 June, compared with previous dates (Figs. 7 and 8) is 
probably related to overnight hypoxia that likely occurred under low-flow conditions observed that day (Fig. 4). 
After 4 June, the community state shifted because of increased abundances of chironomid larvae, Streblospio 
benedicti and Laeoneris culveri (Fig. 9b). Streblospio benedicti appears to have had a recruitment event 
between 4 June and 20 June when total average S. benedicti abundance increased from 1 1,000 to 94,000 m^. 
The recruitment event appeared to be greatest under increased flow conditions. This indicates a possible flow 
treatment and sampling date interaction whereby flow velocity may have affected active substrate selection, 

F-8 V Effects of Temporality, Disturbance Frequency and Water Flom 
on an Upper Estuarine Macroinjauna Community 



passive organisms deposition (Butman 1986a; 1989), or spionid organismal growth (Hentschel 1999a; 1999b). 
The third community state is related to the flood event that began 22 June 1 997. SaUnit)' dropped to %o by 02 
July and had increased to only 2 %o by 01 August. The persistence of low salinity over a period of more than 
one month probably led to declines in total abundance and biomass following the flood (Figs. 7 and 8). For 
example, when salinity was reduced from 19.9 %o to 3.19 %o, S. benedict stzrvived up to 1 1.5 h, whereas P. 
comuta expired within 7 hours, and Leptocheirus plumulosus remained active and normal (Sanders et al. 1965). 
Differential salinity tolerance explains persistence and species changes, and is one component of temporality. 

The strong response of macroinfauna to temporally variable, ambient, environmental conditions rather than to 
experimental manipulations indicates experimental effects were less important than temporal community 
response to natural environmental fluctuations. This is almost certainly true for flow treatment manipulations 
for which no significant effects on macrobenthos were found. Flow effects probably exist, but are barely 
detectable due to environmental variation. Though significant distvirbance frequency effects were found, 
community structure differences were masked by community composition similarities in all treatments within 
collection dates. 

Temporality is the controlling feature of the Rincon Bayou benthic community, explaining most of the variance 
in abundance and biomass (Table 2). Rincon Bayou is an extreme environment, ranging from hypersalinity in 
droughts and freshwater in floods. From 1994 - 1997, salinity at station C ranged between and 160 %o. The 
present study unintentionaDy captured a flood event that led to a sudden drop of salinity from 18 to %o, 
which persisted for more than a month (Fig. 4). Such a salinity change exceeds the tolerances of most 
euryhaline species (Sanders et al. 1965), and led to an increased populations of chironomid larvae and decreased 
populations of Streblospio benedicti and Laeoneris culveri (Fig. 9). In Rincon Bayou, community change 
occurring in response to physical disturbance appears to be limited by natural environmental variation through 
time, which we call temporality. Temporality is simply the short-term analogy to longer-term seasonality. 

Summary 

Succession is controlled more by the natural tempo of environmental variation (e.g., availability of recruits, and 
coincidence of rainfall) than by small-scale events (e.g., patch defaunation and water flow). How appears to be 
a less important determinant of community structure for Rincon Bayou station C compared to natural variation 
of background environmental characteristics (e.g., salinity), but appears to influence abundance, biomass, and 
diversity. Rincon Bayou is generally a low-inflow, microtidal, shallow water habitat subject to broad salimty 
fluctuations concomitant with periodic drought and flood events. During the period of the present study, 
sample collection date was the most important factor in determining community structure indicating 
experimental effects were overwhelmed by natural environmental variation arising from recruitment and flood 
events. Within the context of natural temporal variation, significant differences were found among disturbance 
frequency treatments. Community abundance, biomass, diversity, and evenness decreased with increasing 
disturbance frequency indicating the importance of post-disturbance community persistence in determining 
community structure and succession state, and possible tray effects. Disturbance in the form of flow alteration 
or defaunated sediment increased community abundance and biomass, indicating disturbance may increase 
production of early succession macrobenthic communities in estuaries. 

Acknowledgments 

Funding for research in Rincon Bayou was provided in part by the U.S. Bureau of Reclamation (Grant No. 4- 
FG-60-G4370) and die Texas Water Development Board (Research and Plaiming Fund Contract No. 94-483- 
046) to the University of Texas at Austin, Marine Sciences Institute. Partial support, via the Lund Fellowship, 
was granted by the University of Texas Marine Science Institute. The authors also acknowledge Rick Kalke and 
Robert Biurgess for assistance in the field, and Carol Simanek for data management. 

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Appendix F ♦ F-9 



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Res Board Can 36:1197-1206 



Appendix F ♦ F-11 



Table 1 : Summary of mean community characteristics over all sampling dates for disturbance 
frequency and flow treatments. 



Treatment 
Level 


Abundance 
log(n m ^+1) 


Biomass 
log(gm^+1) 


Diversity 
(N1) 


Evenness 
(El) 


Frequency 










Biweekly 


9.33 


0.64 


1.82 


0.59 


Monthly 


10.37 


1.00 


1.59 


0.37 


Bimonthly 


10.98 


1.27 


1.62 


0.28 


Undisturbed 


9.87 


0.66 


1.31 


0.44 


Flow 










Increased 


10.12 


0.85 


1.54 


0.39 


Decreased 


10.16 


0.90 


1.51 


0.37 


Control 


9.78 


0.66 


1.46 


0.38 



F-12 ^ Effects of Temporality, Disturbance Frequeng and Water Flow 
on an Upper Estuarine Macroinfauna Community 



Table 2: Analysis of three replications of the flow-disturbance frequency experiment. Analyses for total 
experimental data including all dates (3-way incomplete design), when experiments were initiated on two dates (3-way 
factorial design), and when experiments ended on two dates (3-way factorial design) (Fig. 3). Results for a) abundance 
and b) biomass. Table finds degrees of freedom (df), probability level (p), and variance component percentage (%) for 
each main effect. Freq = disturbance frequency, * = interaction, and - = does not exit. 



Treatment 


All Dates 
Incomplete design 


Initiating Dates 
3-way factorial design 


Ending Dates 
3-way factorial design 




df 


P 


% 


df 


P 


% 


df 


P 


% 


a) Abundance 




















Flow 


2 


0.9548 


0.5 


2 


0.1061 





2 


0.0003 


0.2 


Freq 


3 


0.0001 


4.3 


3 


0.0001 





3 


0.0001 


5.0 


Flow*Freq 


6 


0.0001 


2.2 


6 


0.0001 


1.4 


6 


0.0005 


1.4 


Date 


7 


0.0001 


76.5 


1 


0.0001 





1 


0.0001 


86.9 


Flow*Date 


14 


0.0026 


2.0 


2 


0.4542 





2 


0.0033 


1.1 


Freq*Date 


- 


- 


- 


3 


0.0001 


91.9 


3 


0.1339 


0.2 


Flow/*Freq*Date 


- 


- 


- 


6 


0.0171 


1.5 


6 


0.3368 


0.1 


Plot 


1 


0.2443 





1 


0.5136 





1 


0.9747 





Error 


271 


- 


14.5 


118 


- 


5.3 


118 


- 


5.2 


b) Biomass 




















Flow 


2 


0.0672 


1.2 


2 


0.0172 


1.9 


2 


0.0872 





Freq 


3 


0.0001 


12.7 


3 


0.0001 





3 


0.0001 


8.0 


Flow*Freq 


6 


0.0001 


4.6 


6 


0.0001 


3.2 


6 


0.0149 


1.8 


Date 


7 


0.0001 


35.4 


1 


0.1490 





1 


0.0001 


46.8 


Flow*Date 


14 


0.0132 


3.4 


2 


0.9475 





2 


0.0006 


6.6 


Freq*Date 


- 


- 


- 


3 


0.0001 


57.7 


3 


0.0002 


7.5 


Flow*Freq*Date 


- 


- 


- 


6 


0.0042 


10.2 


6 


0.1015 


3.5 


Plot 


1 


0.1170 


0.1 


1 


0.0275 


1.4 


1 


0.8423 





Error 


271 


- 


42.7 


118 


- 


25.6 


118 


- 


25.8 



Appendix F ♦ F-13 



Table 3: Simple main effects for the flow*frequency interaction over all sampling dates. Two-way. randomized 
block ANOVA calculated for each disturbance level for a) abundance and b) biomass Abbreviations as in Table 2. 



Treatment 








Disturbance Frequency Level 












Biweekly 




Monthly 




Bimonthly 


Undisturbed 




df 


P 


df 


P 


df 




P 


df 


P 


a) Abundance 














Flow 


2 


0.1373 


2 


02861 


2 




0.0511 


2 


0.0001 


Date 


2 


0.0001 


3 


0.0001 


1 




0.0001 


7 


0.0001 


Flow*Date 


4 


0.4825 


6 


0.0538 


2 




0.1522 


14 


0.0001 


Plot 


1 


0.4240 


1 


0.9244 


1 




0.4019 


1 


00195 


Error 

b) Biomass 


44 




59 




28 






119 






















Flow 


2 


0.0238 


2 


0.0090 


2 




0.3416 


2 


00001 


Date 


2 


0.1138 


2 


0.0001 


2 




0.0001 


2 


0.0001 


Flow'Date 


4 


0.3841 


4 


0.5400 


4 




0.0168 


4 


0.0001 


Plot 


1 


0.7145 


1 


0.3949 


1 




0.1490 


1 


0.0645 


Error 


44 




44 




44 






44 





F-14 V Effects ofTemporality, Disturbance Frequency and Water Flow 
on an Upper Estuarine Macroinfauna Community 



Table 4: Simple main effects for the flow'frequency interaction over all sampling dates. Two-way, 
randomized block ANOVA calculated for each flow level for a) abundance and b) biomass. Abbreviations as 
in Table 2. 



Treatment 




Flow Treatment Levels 






Control 


Increased 


Decreased 




df 


P 


P 


P 


a) Abundance 










Frequency 


3 


0.0001 


0.0001 


0.0001 


Date 


4 


0.0001 


0.0001 


0.0001 


Freq*Date 


6 


0.0001 


0.0001 


0.0001 


Plot 


1 


0.4117 


0.5310 


0.5807 


Error 


69 








b) Biomass 










Frequency 


3 


0.0001 


0.0001 


0.0033 


Date 


4 


0.0001 


0.0001 


0.0006 


Freq*Date 


6 


0.0505 


0.0001 


0.1453 


Plot 


1 


0.0593 


0.9175 


0.9739 


Error 


69 









Appendix F ♦ F-15 



Flow Treatment 



Two plots per flow 
treatment 



Increased (Decreased or Control) 



Disturbance 

Frequency 

treatments 



Collection 
Date 




Same as for 1 



Biweekly Monthly Bimonthly Undisturbed 



5 8 





12 3 4 5 6 7 8 



Sample 

Replicates .-, , 

(3 per date) ' " -^ l 2 3 l 2j* I z J 



I 



1 23 



Figure 1 : Experimental desigr^ of the study. The experiment was a three-way, randomized block, 
incomplete factorial design to evaluate effects of flow regime and disturbance frequency on macrobenthos 
over time. Flow treatment levels were: increased by weirs and decreased by nets relative to controls with no 
structure. Disturbance frequency treatment levels were: biweekly = every 2 weeks, monthly = every 4 
weeks, bimonthly = every 8 weeks, and undisturbed. Collection dates were: 1 = 23 April 1997, 2 = 07 May 
1997, 3 = 22 May 1997, 4 = 04 June 1997, 5 = 20 June 1997, 6 = 02 July 1997, 7 = 17 July 1997, and 8 = 01 
August 1997. 



Predominant Water Flow 



a) 




Control 2 



Net 2 



Direction of 

wind and tidal 

flow 



Weir 2 



) ( ^^ 



Control 1 



Weir 1 



/ \ Net 1 



b) 



Figure 2: Flow alteration treatments for a) weir and net structure designs and b) field emplacement. 

Trays of defaunated sediment were randomly placed in hatched center regions. Undisturbed core samples 
were obtained in the shaded regions. Control plots were defined by corner posts (dotted lines). 



F- 1 6 V Effects ofTemporality. Disturbance Frequency and W''ater Flow 
on an Upper Estuarine Macroinfauna Community 



Biweekly Monthly 



Bimonthly 



23 April 


r 1 


u 


V 


07 May* 






22 May 


> 






04 June 


• 2 mi 


1 


20 June 


Y3 


Y3 I3 


02 July 


^ f 




17 July* 


• 

Y4 








01 August 


\f4 


> 



Figure 3: Time line for sample collection dates in 1997. Tray placement indicated by dots (#) and 
sample collection indicated by arrow heads (>■). Date flow velocity measurements were taken indicated by 
asterisk (*). Beginning of the flood event (22 June) indicated by dashed (-) line. Disturbance frequency 
treatments were replicated twice, two different ways: by same initial dates (dots 1 and 2) and same ending 
dates (dots 3 and 4). Undisturbed samples were collected on every date. 



Appendix F ♦ F-17 




Figure 4: Hydrographic conditions over sampling period. Average salinity, dissolved oxygen, water 
temperature, and turbidity measured each sampling date. Standard error bars are smaller than symbols. 
Minor ticks placed every Monday. Daily rain fall and inflow from gage (USGS 1997). 





150 - 


^ 


r 


iv 


^_^ 


125 - 




: 


I 


i 




( 


► 




1 


100 - 


5 


I 




O 


75 - 






o 


^ 


50 - 












o 




• 


Control 




PL, 


25 - 




o 


Decreased 










- 




▼ 


[ncreased 




1^ 


^ 












X 




07 r 


1 


1 
17 Jul 










19 


97 







Figure 5: Average flow velocity measurements from flow treatments on two dates (7 May and 
17 July 1997). Tukey minimum significant difference is 26.45 for flow regimes, and 18.29 for dates. 



F-1 8 ♦♦♦ Effects ofTemporality, Disturbance Frequeng and Water Flow 
on an Upper Estuarine Maavinfauna Community 



14.5 



V. 14.0 ] 

B 

GO 

O 

? 13.0 

I 
"^ 12.5 



c 

5 



12.0 



3.5 



+ 3.0 
I 

s 

^ 2.5 



I 2.0 



1.5 



-•— Control 
-O— Decreased 
-▼— Increased 



b) 





.eV\^ <v^^ ^^"^^ x<0^^ 

Disturbance Frequency 
Figure 6: Disturbance frequency*flow interaction for average a) abundance and b) biomass. 



Appendix F ♦ F-19 



+ 



13 



12 



11 



^ 10 

o 

c 

-S 9 

c 



2.5 



1/5 

E 
o 



-•— Control 
O Decreased 
-▼— Increased 




-•— Control 
O Decreased 
-T— Increased 




Figure 7: Flow'date interaction for average a) abundance and b) biomass (b) in undisturbed 
sediment. Minor ticks placed every Monday. Tukey minimum significant difference is 0.20 for abundance 
and 0.12 for biomass. 



F-20 V Effects ofTemporality, Disturbance Frequency and Water Flow 
on an Upper Hstuartne Macroin/auna Community 



+ 



T3 
00 

o 
o 

c 

c 

3 
< 



13 
12 
11 
10 
9 
8 



2.5 



+ 



.s 

GO 

o 



«0 

I 



— • — Undisturbed 

V Biweekly 
— ■ — Monthly 
— O Bimonthly 




i 



a) 




T" 



-• — Undisturbed 
V Biweekly 
-■ — Monthly 




Figure 8: Disturbance frequency*date interaction for average a) abundance and b) biomass from 
control flow plots. Minor ticks placed every Monday. Tukey minimum significant difference is 0.59 for 
abundance and 0.25 for biomass. 



Appendix F ♦ F-21 



<N 




PCI 



(N 




PCI 



Figure 9: Principal component (PC) analysis for all species data. A) Plot of PC scores for dates: A = 
23 April 1997, B = 07 May 1997. C = 22 May 1997, D = 04 June 1997, E = 20 June 1997, F = 02 July 1997, 
G = 17 July 1997, and H = 01 August 1997. B) Plot of standardized PC loadings for species: CL = 
chironomid larvae, LC = Laeoneris culveri , ML = Mulinia lateralis . PC = Polydora cornuta , and SB = 
Streblospio benedicti . 



F-22 ♦♦♦ Effects of Temporality, Disturbance Frequency and Water Flow 
on an Upper Estuarine Macroinjauna Community 



APPENDIX G 



Field Notes and Observations 
from Benthic Sampling Trips: 
October 1994 - December 1999 



In eady October 1994 Paul Montagna, Rick KaUce, Teny Whitledge, Dean Stockwell and Ken Dunton met with 
Mike Irlbeck at Rincon Bayou to discuss station locations. The road and the pasture to Stations A & B were 
very wet and the water level was near the top of the culverts and flowing from A to B. The secondary diversion 
channel was not finished but a ditch cut through the road in its place restricting travel. 

SAMPLING DATES 

28 Oct. 94. First benthic sampling trip. Salinity was low from previous inflow event, Rt^ppia was noted in the 
reference sites A & B. High D. O. at A & B associated with the Ruppia. Roads were muddy. 

Note: Rsference sites A<&B don't appear to he affected by daify tidal movement as an sites. C, D, E and F in Rincon Bayou. 

11 Jan 95. Benthic samples. Low tides at stations in Rincon Bayou. SaUnity normal, no signs of increase with 
low water. 

12 Apt. 95. Benthic samples. Salinity normal. Ruppia very short and sparse at A & B. We used push-net at 
Station C for isotope samples. Lot of mysids, grass shrimp, and some brown shrimp. Water level up with 
spring high tides. 

12 July 95. Benthic samples. Summer low tides, hot dry conditions with high evaporation. Road and pond at 
Stations A & B covered with white pelicans. Ibis, Stilts and mottled ducks. Lot of terns s kimmin g surface. 
Low oxygen levels. Some dead Cjrprinodon variegatas and a few blue crabs along shoreline. Some live crabs 
along shore at air/water interface, fish appear to be gulping for air, and numerous corixids (water boatmen) at 
A & B. Salinities high at all stations. Water levels down in Rincon Bayou at Stations C, D, E and F. Dead crab 
and Cyprinodon at Stations C & D. Station E, mud wet but no standing water. Approximately 40 yards out 
water 0.01 m deep , dug hole for hydros, cores from wet mud. Saw a live Laeoneneis culveri on surface of mud. 
80.5%o pore water and same on surface water. Salt crystals on svirface of adjacent algae flat Station F, sure is 
hot, water low 15-20 ft. from shoreline. Station G (Railroad Trestle) salinity was 43.5%o. 

3 Oct 95. Benthic samples. Water level was high due to Hurricane Opal in Gulf of Mexico. Tidal flushing 
lowered salinities. Stations A & B water level up, oxygen OK, salinity down a littie. Stations C, D, E & F 
flushed from high tides. Isotope sample from Station G. 

31 Oct. 95. Exploratory trip. Dean Stockwell and Rick Kalke. Nueces River Diversion Channel just 
completed. On 29 Oct. 95 Corpus Christi received 3"^ highest record rainfall, 10+ inches over-night. We were 
able to motor with the jet boat from Nueces River to C-5 the culvert between the river and Station C. Mike 
Irlbeck had hiked in from the road and we gave him a ride out. Salinity at the culvert was 0.57%o. We drove to 
Stations A & B by vehicle. High water due to run-off and tides. Surface water was flowing from B to A 
because of the wind and the salinity was 10.16%o. Large mats of blue -green algae were on the surface and along 
the shore. Took slides. Roads too wet to go to other stations. 

13 Dec. 95. Isotope samples from the mouth of Rincon Bayou (L=50, C, F, H = 68). 



Appendix G ♦ G-1 



8 Jan, 96. Benthic samples. Winter low tides evident at Stations E & F. Salinities in Rincon Bayou in the 30's 
to 39.0%o at station F. 

21 Feb. %. Isotope samples. Water very low. Stations A & B. .1 m deep, salinity 33. 3%o. Rincon Bayou 

Station C ~ .05 m deep 81%o, D & E no water. Station F only puddles, refractometer 122%o. Highest 
salinities, beginning of 1996 drought? 

19 Mar. 96. Isotope samples. No water at Stations A, B, C, D & E. ~ .05 m water at Sta. F, 68.6%o. 

9 Apt. 96. Benthic and Isotope samples. At Station G- Railroad Tresde. Water still low, sampled lake east of 
railroad tracks ~4" deep, very turbid, collected some post larval brown shrimp for isotopes. Station A- dr)», 
only a puddle in center "- l"deep, algae layer on top of mud. Station B, sample area diy except for water 
between cracks in mud (water from rain fall from previous week) 50%o. Station C, dr}', only damp mud, only 
water was in cracks and in wash-out area around posts, 98%o. Station D, no standing water, a little water in 
cracks and animal tracks, 100%o. Station E, 1/4 inch water from where cores were taken, foot tracks from Jan. 
98 still there, 82%o. Station E, removed mud from same depression as Jan. 96 for Hydrolab, very low water 2- 
3" deep, 59.3%o. Station H=68 Nueces River Bridge, 6.8%o. 

13 May 96. Isotope samples. Water level back up again but salinity levels stiU high. Station C, water ver^' clear, 
bottom covered with blue green algae mat (very green) - sampled with push-net - no sign of any shrimp only a 
few Cyprinodon, 59.3%o. Sta. F, 51.8%o, water very turbid, lot of small swirls from fish and shnmp, some port- 
larval brown shrimp collected. Station G, (Tresde) 51.7%, water turbid, brown shrimp numerous, saw redfish 
feeding so shallow its back was out of the water. Station H = 68-river 8.9%o, collected brown shrimp and blue 
crabs with push-net. 

1 June 96. Isotope samples. Station H=68, 9.9%o collected brown shrimp, spot and Rangia. Station C, 70.4%o, 
water from high tide was flowing through upper culverts C-5 & C-4 through diversion channel to the Nueces 
River. 

12 July 96. Benthic samples. Stations A & B, no water, took cores on dry land, very Uttie moisture in sediment. 
Stations C & D. Small amount of water, saltnit)' C=159.2%o, D=120%o. Stations E & F water shallow, 
85.86%o, no shrimp, picture of algal mats on slides. Station G=Trestle, 84%o no shrimp. Station H=68 River, 
2.1%o, push-net for shrimp, caught 1 brown shrimp, saw a couple of large shrimp avoiding net. 

22 Oct. %. Benthic samples. Stations A & B - water back ~ .1 m, 40%o. Stations C, D, E and F - water back, 
evidence of lower salinity input from the River Diversion Chaimel 25.1%o at C-* 46.4%o at F. 

6 Jan. 97. Benthic samples. Stations A & B, bottom covered with dense bed of Ruppia. 52.4%o, Salinity 
gradient from C to F = 25.0%o to 56.4%o. Stations E and F sediment surface with layer of algal mat. 
Station H = 68 River 0%o. Low flow coming over dam. 

23 Apt. 97. Benthic samples. Stations A & B, low oxygen, water a thick green color, Kuppia seems to be 
declining, not as thick as Jan. 97. Stations C, D, E, and F, salinity down to 15-16%o. Station D, lot of medium 
to large blue crabs and some reds and drum. Station E, water shallow 2-3 in., slight outgoing current 1045, algal 
mat over most of bottom, took benthic samples from bare bottom. 

2 July 97. Benthic samples. Stations A & B, salinity down to 4.8%o, water up, Ruppia covering most of bottom, 
very healthy, to surface of water. Station C, water ~ 1 ft deep at shoreline, evidence of being higher, Christine's 
experimental plot completely under water. Salinity from C to F was 0.2 to 4.4%o. Station H=68 at river 0%o. 
Station 62 Whidedge station in flats north of secondary' diversion channel, puddle at culvert was 0%o, water in 
flat on way to 62 was 4%o and at 62 stake 2%o Station D, washout in culvert on road. 

28 Oct. 97. Benthic samples. Light rain, low salinity at all stations. Stations A & B, 4%o C to F, 0.3 to 4.1%o. 
Water level down from the flood event in July 97. 



G-2 ^ Field Notes and Observations from Benthic Sampling Trips 



16 Jan. 98. Benthic samples. Stations A & B hydrogtaphic conditions good. The area between Stations C & D 
seems to pond during low tides. In the summer long term ponding will sometimes evaporate and dry up. 
Station C was 0.1 m deep and Station D was 0.04 m deep. Water level was low, only small ditch of water form 
culvert between upper and lower Rincon Bayou. Hole in road covered with pile of fill. Station E, no water, 
only damp mud with numerous worm casts. Station F, dry, only wet mud with occasional puddles, 35%o. 

8 April 98. Benthic samples. Station A, short Ruppia. Station B, surface of sediment with ~ 4cm loose flock 
(mud), 16%o. Station D, water level up, C-5- water just to culverts with small channel or trickle of water 
moving. Station D, large number of black drum feeding in area aroimd D, water flowing out toward bay. 
Salinity from C to F was 12.9 to 30.2%o. 

9 July 98. Benthic samples. Salinity, high 104%o. Station A & B, very low water, oxygen low, lot of wading 
birds and sea-guUs apparently feeding on Cyprinodon variegatas, etc. Stations C-F, water level normal, not low. 
Station D, lot of larger fish, reds and drums. Salinity high C to D, 60%o. 

5. Aug. 98. Primary production. Rick KaJke with Kevin Neely. Stations A & B, dry, no water. Station C, salt 
deposits on surface, only sheen of water on mud surface. Station D, basically dry, only narrow strip of water. 
Stations 67 & 62, dry. Station F=61, 94.93%o, Station 83, 50.2%o, Station 60, 70.24%o. 

Starting in September 1998, 1 (Rick Kalke) began sampling with Ijynn Tinnin and Terry Whitkdge 's primary production 
processes. 

29 Sept. 98. Primary production. Station A & B, water level up, 28.2 - 28.4%o. Stations C & D, salinities 
indicate inflow event, 2.3 to 5.6%o, current going out of Rincon through culvert toward bay to Stations E & F, 
22.1 and 20.8%o. Station H=68; river 0%o. 

28 Oct. 98. Benthic samples and collected water for Lynn Tinnin, not able to get water for primary production 
samples. Major flood occurred on 17 Oct. 98. Water level had been high enough to wash out culverts and road 
at the secondary diversion channel and washed a large portion of road out at Station D. We had to hike in 
from the secondary wash-out to collect benthic and water samples from Stations D, E and F. Stations A & B, 
water up, running from A to B, 2 & 1.5%o. Stations C-F , major freshwater flushing to 0.8%o. 

TDite to road wash-outs and I or wet conditioru we were unahk to sampk some of the lower Rincon Ba^ou Stations, i.e. Station D, 
E, and F until 24 Feb. 99. 

18 Nov. 98. Primary Production. Stations A & B, water flowing from A to B, salinity 3.8 to 3.7%o. Station C, 
water high, up to grass line along shore, about same in Oct. 98. Road real wet in spots, maybe a Uttie drier than 
Oct. Station H=68, river up over sides of boat ramp to shoreline, 0%o. 

17 Dec. 98. Primary Production. Stations A & B, current from A to B, 14.6 and 14.7%o. Station C, 11. 7%o, 
roads still washed out. Station H=68, river level back down to bottom of boat ramp. 

12 Jan. 99. Benthic samples. Stations A & B, 10.1 to 13.8%o, lot of water boatmen (corixidae) Stations C & D, 
water in upper Rincon ponding 19.3%o & 19.9%o. Station E & F, had to hike in from Station D, low water in 
their area, salinity 25 & 23.8%o. 

13 Jan. 99. Primary production. Stations A & B, salinity 10.8%o & 12.3%o. Station C, water golden brown in 
color, 19.9%o. Station H=68, river 0%o. 

24 Feb. 99. Primary production. Stations A & B, water turbid, 15%o. Station C, salinity, 33.97, higher than 
Stations D, E, and F. Water at Station D turbid brown color, flowing into Rincon, 2" deep in ctilvert. Station 

E, algal flat dry, Salicomia patches show fresh growth. Station F water to edge of grass and clear. Station H=68 
salinity 1 .45, no flow over dam. First Nueces River Diversion Channel flow gage information. 

18 Mar. 99. Primary production. Stations A & B, water turbid 20%o. Stations C & D, salinity down to 19.33 & 
28.85. Station E, high water line was up to power poles but currently at shore Une. Algal flat inactive. Station 

F, water turbid, up to grass line. Station H=68, river 1.25%o, no water over dam. 



Appendix G ♦ G-3 



14 Apr. 99. Benthic samples. Stations A & B water up, .35m depth, salimty 13.78%o. Station C, salinity 4.84%o 
from diversion channel. Station D, water very turbid, 15.78%o, some large fish swirls probably reds, drum and 
mullet. Stations E & F water .27 & .35 m, large school of redfish at Station F, tails out of water (see slide). 
Station G, tresde, 21.86%o slight out-going tide. 

15 Apr 99. Primary production. Conditions similar to 14 Apr. 99 except wind much stronger, NW-20 MPH. 
Water flowing over dam at Nueces River. Station H=68, 0.44%o. 

24 May 99. Primary production. Stations A & B, dense beds of Bjtppia, salinity, 16-18%o. Stations C & D, 
water muddy, lot of fish activity at both sites, redfish at Station D, caught some brown shrimp with push-net at 
Station C. Station E & F, lot of small fish, shrimp and redfish activity. Station H=68, river 2.02%o, water 
seeping over dam, no flow. 

9 June 99. Primary production. Stations A & B, dense beds of Ruppia up to surface, low ox}'gen 1 .95 to 1 .49 
ppm (mg/1), salinity 24-25%o. Stations C & D, 20 to 35%o. Station E, 39%o, at least 1 redfish and lot of bait- 
fish. A little water in algal flat was 1 10%o, dead Cyprinodon and brown shrimp 8 to 9.5 cm long in algal flat 
area. Probably trapped in evaporating pools left by low tides. Station F, lot of mullet and small bait swirls and 
a few redfish. 

7 July. 99. Benthic samples. Stations A & B, salinity down to 8.3%o, oxygen low 1.59 to 1.7 ppm. (mg/1), 
Ruppia present but not as thick as it was in May and June. Freshwater inflow event, salinity down at Stations C- 
F, to 1.3 to 6.3%o. Rattiesnake at road right before Station D. Strong flow of freshwater from Station D to E 
& F. Station E, algal flat 10%o, algal mat green near shallow edges, gray and floating in deeper areas, no mats 
visible at Station E. 

21 July 99. Primary productions. Stations A & B, oxygen low 1.6 ppm (mg/1) depth .21 to .27 m. Stations C 
to F, salinity still low 4.2 to 13.6%o. Station C oxygen 3.2 ppm. Station H=68, river, 0.6%o. 

19 Aug, 99. Primary production. Stations A & B, water diytng up, dead blue crabs in standing water, few live 
crabs along edges of bank out of water to get air, lot of shore birds in what litde water left. No water where we 
normally sample, water taken by culvert for A & B. Stations C & D, water low, salinity 26 to 45%o, lot of 
wading birds, ~ 6 dead black drum carcasses on shore. Station E, no water ("I told you so!") says Ms, Tinnin. 
Station F, .05 m, 39%o. Station H=68 river, 0.48%o, barely flowing over dam. 

16 Sept. 99. Primary production. Stations A & B, water back as result of rain and tides from hurricane Bret. 
Ruppia starting to grow along shorelines. Station C to F salinity 0.93 to 8.51%o. Station D current coming in 
from the bay, blue -green algal floating on surface, lot of fish activity, water turbid. Stations E and F, lot of fish 
and mullet, reds, shrimp. Algal flat at Station E, 2%o. Blue green algal floating at Stations E & F. Station 
H=68, river 0.43%o, flow over dam. 

28 Oct. 99. Benthic and primary production samples. Stations A & B, Ruppia growing along shoreline. Station 
C, lot of white shrimp, white pelicans and avocets. Station D, water real low, ponding between C & D, white 
shrimp jumping by culvert. Stations E & F, 16 & 15%o, Station H=68, river 1.22%o, no flow over dam. 

17 Nov. 99. Primary production. Stations A & B water soupy green, 15 to 14%o, Ruppia along shoreline. 
Stations C & D, water clear 1 1 to 1 9%o, numerous bird tracks, no shrimp. Station E, water clear 25%o, Station 
F, SE wind began blowing and quickly stirred up sediment, now very turbid. Station H=68, river 0.85%o, no 
flow over dam. 

8 Dec. 99. Primary production. YSI instrument not working. Only refractometer for salinity. Station A & B, 
15 to 14%o. Station C, D, E and F, 10, 12 and 25%o. Station H=68 0%o, scattered showers. 



Nueces River Diversion Channel Water Exchange 

At the February, 1 999 Rincon Bayou Demonstration Project team meeting there was some concern in the 
accuracy of the exchange rates of water moving through the diversion channel into and out of upper Rincon 
Bayou. During at least six sampling trips to Rincon Bayou we checked the hydrographic conditions, the flow 

G-4 ^ Field Notes and Observations from Benthic Sampling Trips 



24 Feb. 99 


1230 


18 Mar. 99 


1200 


14 Apr. 99 


1410 


15 Apr. 99 


0800 


24 May 99 


1210 


9 June 99 


1205 



1.1 


2.04 


1.78 


1.54 


1.2 


1.67 



patterns through the diversion channel, at the Rincon Bayou Station, 08211503, at the culverts above station C, 
station D and the culvert at this site, stations E and F and when available stations 50 and 51 at the mouth of 
Rincon Bayou in Nueces Bay. Observations at Station 08211503 verified with the gage data for this station 
indicate that the water exchange reported from this gage is accurate. The following is an example of the staff 
readings and the gage read-out for 6 observations. 

DATE TIME GAGE STAFF 

1.14 

2.15 

No Data 

1.57 
1.23 
1.69 

The daily flow regime through the Nueces River Diversion Channel into Rincon Bayou is now established. The 
connection between the channel and upper Rincon Bayou where the ranch road crosses has resulted to a 
narrow ditch where water exchanged takes place. The ditch has vegetation growing up in both banks and 
seems to be maintained by almost daily tidal exchange into and back out of upper Rincon Bayou. Although no 
diel observations were made, the wet banks near low areas suggest tidal inundations moves water onto the flats 
occasionally. There is a split of tidal flow in the upper Rincon with tidal movement coming and going from 
both Nueces Bay and the Nueces River, resulting in higher salinity estuarine water from the bay meeting with 
lower salinity Nueces River water from the diversion chaimel in the upper Rincon Bayou. This exchange has 
helped alleviate the stagnated high salinity conditions observed when upper Rincon Bayou was a dead-end. 
High salinity conditions can still be expected during droughts and extreme low tides especially dimng the 
summer. The extent of water exchange in Rincon Bayou depends on the magnitude of the tidal of freshwater 
inflow event. A major event results in complete flushing and mixing of the system all the way to the bay while 
normal daily tidal movement may only move enough water to maintain some exchange with upper Rincon 
Bayou which is important. 

Rincon Bayou Seasonal Observations 

Hydrographic and environmental conditions at Rincon Bayou are dependent on annual tidal cycles and seasonal 
climatic conditions. Local weather events can dramatically effect these conditions, i.e. droughts, floods and 
tropical disturbances. 

The typical annual cycle based on annual quarters: 

January usually results in low winter tides and cool to cold temperatures. These conditions don't typically result 
in high evaporation and high salinities. 

April conditions are associated with spring high tides which disperse brown shrimp post larvae and crab larvae 
throughout the delta and niu-sery areas. Many drum and redfish migrate into the area to feed on the growing 
shrimp population. 

July often has periods of very low tides, low rainfall, associated with high temperatures and high evaporation. 
Rincon Bayou stations often dry up or the water becomes hypersahne. 

October is the transition for summer to fall. The tides may be high especially with tropical storm activity and 
flooding may occur modifying typical high salinity conditions from summer. At the control stations A & B 
which are often dry in July and August the fall rainfall results in the germination and growth of Ruppia beds 
which are an important food source for waterfowl. 



Appendix G ^ G-5 



United States Department of the Interior 
The mission of the Department of the Interior is 

to protect and provide access to our Nation's 

natural and cuUural heritage and honor our trust 

responsibilities to tribes. 

Bureau of Reclamation 
The mission of the Bureau of Reclamation is to 
manage, develop, and protect water and related 

resources in an environmentally and 
economically sound manner in the interest of 





P00001884